Thyroid Cancer: Symptoms, Types, Rates in Japan and Abroad, the Effects of Iodine (Kombu) and Radiation, and Thyradin, TSH, Thyroglobulin and Tumor Markers

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Please note This article organizes what public agencies and medical societies have written. It cannot make a diagnosis or decide treatment or drug doses. If you have questions about your test results or treatment, be sure to talk to your doctor.

Thyroid cancer is a cancer about which many things are said: “more people are being found,” “eating too much kombu is bad,” “radiation increases it.” People who have had surgery often ask: “What is the hormone drug (Thyradin, チラーヂン) made of?”, “My TSH went up after one side was removed. Does that make recurrence more likely?”, “My thyroglobulin went up and I’m worried,” and “Does CA19-9 have anything to do with it?”

This article checks the following against national statistics, Japanese and U.S. clinical guidelines, and papers:

• How the thyroid works, and how TSH (thyroid-stimulating hormone) works
• The types of thyroid cancer
• Incidence in Japanese and non-Japanese people, and how it has changed over time
• The effects of food (especially iodine, which means kombu) and radiation exposure
• The ingredients of Thyradin-S, and what taking it does to the body
• Whether taking Thyradin for a long time damages the kidneys or other organs
• TSH and recurrence after one side is removed, rising thyroglobulin, and tumor markers (including CA19-9)

Conclusions

• About 90% of thyroid cancers are papillary cancer, a cancer that often progresses slowly. In Japan, the 5-year relative survival rate is 94.3% (people diagnosed in 2018)
• The number of people diagnosed with thyroid cancer in Japan rose from about 1,700 in 1975 to 16,505 in 2023. Meanwhile, the share of people dying from it (the age-adjusted mortality rate) has not increased. In South Korea, the number of people found at one point rose to nearly 10 times as screening spread, and the International Agency for Research on Cancer (IARC) estimates that much of this increase is overdiagnosis (finding even small cancers that would never have caused trouble in a person’s lifetime)
• By country, the rate at which cases are found in South Korea and China is more than 3 times Japan’s, but mortality is about the same everywhere, 0.2 to 0.6 per 100,000 people
• As for food, iodine (abundant in seaweed such as kombu and hijiki) can lead to thyroid disease both when there is too little and when there is too much. One Japanese study found more papillary cancer in postmenopausal women who ate seaweed every day, but no food has been rated “convincing” in international assessments. What is clearly linked is body fat (obesity)
• For radiation, it is established that exposure in childhood increases thyroid cancer (atomic bomb, Chernobyl). For Fukushima, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) and the Fukushima Prefecture evaluation subcommittee say no link between the thyroid cancers found and radiation exposure is “recognized.” For exposure in adulthood, even studies of atomic bomb survivors have found almost no increase
• The active ingredient of Thyradin-S is synthetic levothyroxine sodium, which acts the same way as the body’s own thyroid hormone (T4). The tablets contain no lactose as an additive. Too high a dose can cause effects such as palpitations, arrhythmia and weaker bones
• No report was found of long-term Thyradin use damaging the kidneys. Kidney disease is not among the side effects in the package insert, and in fact too little hormone makes a kidney test value (creatinine) worse. What to watch for with long-term use is the heart and bones when the dose stays too high, and rarely liver injury
• Whether a high TSH after one-sided removal increases recurrence is a question on which studies disagree, and it is not settled. The Japanese guideline (2024) says TSH suppression “should not be done” for people who had one side removed, and the U.S. guideline (2025) uses keeping TSH “within the reference range” as the target
• Thyroglobulin after one-sided removal is also made by the remaining thyroid, so the number alone cannot tell you whether the cancer has recurred. The Japanese guideline says “it is good to measure it over time,” while the U.S. says “routine measurement is not recommended,” so Japan and the U.S. differ
• CA19-9 is not a standard tumor marker for thyroid cancer. It has been reported to rise in some advanced medullary cancers, but it also rises with diabetes and with bile duct and lung diseases

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How the thyroid works

A butterfly-shaped organ below the Adam’s apple

The thyroid is a small organ weighing about 10 to 20 g, located just below the Adam’s apple and wrapping around the front of the windpipe. It is shaped like a butterfly with its wings spread, made up of right and left “lobes” and a central “isthmus” (Cancer Information Service).

Just behind it are the recurrent laryngeal nerve, which moves the vocal cords, and the parathyroid glands, rice-grain-sized glands (four, upper and lower, left and right) that regulate calcium in the blood. This is why thyroid surgery can sometimes cause hoarseness or lower calcium (Japan Thyroid Association).

The hormones the thyroid makes

The thyroid mainly releases T4. In the body, T4 loses one iodine to become T3, and acts in that form. Of the T3 in the blood, only about one-fifth comes directly from the thyroid; the rest is made from T4 (MSD Manual Professional Edition).

(From the Cancer Information Service, “About thyroid cancer”; the Japanese Association of Clinical Laboratory Specialists, “Thyroid tests”; and the American Thyroid Association, “Thyroid Function Tests.”)

How TSH (thyroid-stimulating hormone) works

The amount of thyroid hormone is regulated by TSH (thyroid-stimulating hormone), which is released by the pituitary gland in the brain.

• The hypothalamus in the brain releases TRH, and in response the pituitary releases TSH. TSH tells the thyroid to “make hormone”
• When thyroid hormone in the blood is sufficient, a brake is applied to the hypothalamus and pituitary, and TSH falls. When it is insufficient, TSH rises. Because of this relationship, TSH is the best guide to whether thyroid hormone is too much or too little
• TSH also stimulates thyroid cells to grow. The Cancer Information Service explains that it “also stimulates thyroid cancer cells and has the effect of increasing cancer cells.” This is the reasoning behind the approach of keeping TSH low in thyroid cancer treatment (TSH suppression therapy)
• The reference range of TSH for Japanese adults (ages 20 to 60) is 0.61 to 4.23 mIU/L (Japanese Society of Laboratory Medicine, 2020; values adjusted for differences between test kits). A reference range is “the range that contains 95% of healthy people,” so even 5% of healthy people fall outside it

Types of thyroid cancer

Thyroid cancer is divided into types by the shape of the cancer cells (histologic type), and how fast it progresses differs completely from type to type.

(From the Cancer Information Service and the Clinical Practice Guidelines for Thyroid Tumors 2024. Malignant lymphoma can also occur in the thyroid, but it is treated as a separate disease because its origin and treatment differ.)

• Papillary cancer and follicular cancer together are called “differentiated cancer”
• In the U.S., papillary cancer is said to be 70–80% and follicular cancer 10–15% (American Thyroid Association), so Japan has fewer follicular cancers
• The 2022 revision of the WHO classification created a category of “low-risk tumors that cannot be called either malignant or benign (such as NIFTP).” Japan’s “General Rules for the Description of Thyroid Cancer, 9th Edition (2023)” has adopted it as well

Symptoms

According to the Cancer Information Service, thyroid cancer often causes no symptoms. Even when there are symptoms, almost none are seen other than a lump in the neck.

As the disease advances, symptoms such as the following can appear:

• A feeling of discomfort or pain in the throat
• Hoarseness (dysphonia)
• Difficulty swallowing, or aspiration (food or drink entering the windpipe)
• Blood in the sputum (hemoptysis)
• Shortness of breath

The Cancer Information Service advises seeing a family doctor or an ear, nose and throat doctor early if you notice any symptoms. Screening of people without symptoms is covered below in “The official position on screening.”

Survival

(Cancer Information Service, National Cancer Center Japan, “Cancer Statistics by Cancer Type: Thyroid,” updated July 2026)

For papillary cancer, age matters a great deal. In a table in the guideline that pooled results from four Japanese facilities, 20-year disease-specific survival is 98.4–99.8% for low-risk people and 54.0–82.4% for high-risk people aged 55 and over.

Incidence in Japanese and non-Japanese people, and how it has changed

Japan: more people are found, but deaths have not increased

• The number of people diagnosed with thyroid cancer went from 1,691 in 1975 to 16,505 in 2023 (4,445 men, 12,060 women). Women are about 2.7 times as many as men
• The age-adjusted incidence rate (per 100,000 people) for women also rose about 6-fold, from 2.7 in 1975 to 16.8 in 2019
• Meanwhile, 1,887 people died in 2024 (597 men, 1,290 women), and the age-adjusted mortality rate for women has actually fallen, from 0.9 to 0.5
• By age, women begin to increase from ages 15–19 and peak at 70–74; for men the peak is 65–79 (Guideline 2024, 2019 data)

By country: the number found differs 6-fold, but mortality is about the same

South Korea (32.0) and China (30.6) are more than 3 times Japan (8.8), but mortality is 0.35 in Japan, 0.32 in South Korea and 0.48 in China, all within the range of 0.2 to 0.6. Worldwide, an estimated 960,000 people were diagnosed with thyroid cancer in 2024.

What happened in South Korea

• In South Korea, government subsidies for cancer screening began in 1999, and thyroid ultrasound spread. The thyroid cancer incidence rate in 2011 was 15 times that of 1993, but most were small papillary cancers under 1 cm, and mortality did not fall (Guideline 2024, Ahn 2014)
• Of the increase from 1999 to 2008, 94.4% was cancers smaller than 2 cm (Park 2016)
• In 2014, doctors issued a statement warning of overdiagnosis, and as news coverage spread, thyroid cancer incidence in South Korea fell by about 30% in 2014 (Lee 2020)
• In a Korean case-control study, thyroid cancer deaths among people who had been screened did not differ from those among people who had not (odds ratio 1.13, 95% confidence interval 0.49–2.63)

What “overdiagnosis” means

In 2016, IARC used data from 12 countries to estimate the share of women’s thyroid cancers in 2003–2007 that were overdiagnosis: about 90% in South Korea, 70–80% in the United States, France, Italy and Australia, and about 50% in Japan, the Nordic countries and the United Kingdom (IARC press release No. 246). A 2020 follow-up said that more than 1 million people in 26 countries may have been overdiagnosed in 2008–2012 (No. 284).

The supporting evidence is the “small papillary cancers that were not noticed during life” found in autopsies of people who died.

• In a pooled analysis of 35 studies (about 13,000 people), they were found in 11.2% when the thyroid was examined closely, and that share has not changed since 1970 (Furuya-Kanamori 2016)
• The Japan Thyroid Association explains: “Including very small ones, they are found in about 10% of people. That frequency has not changed from the 1970s to the present”
• Depending on how the examination is done, the range runs from a few percent to the 30s of percent (28.4% in a 1975 Japanese report)

In other words, many people have always had “small papillary cancers that would never trouble them in their lifetime” in the thyroid, and the more ultrasound spreads, the more of them are found. That is the structure.

The official position on screening

Cases with symptoms such as a lump in the neck or hoarseness are different. If you notice any symptoms, see a doctor.

For small papillary cancers, “not operating right away” is also an option

For papillary cancers that are 1 cm or smaller, have no lymph node or distant metastasis, and are away from the windpipe and nerves (very low risk), active surveillance, which means not operating right away but monitoring with regular tests, is “recommended” (strength of recommendation: strong) in the Japanese Guideline 2024.

• It is a method that began in Japan, started by Kuma Hospital (Kobe) in 1993 and by the Cancer Institute Hospital (Ariake) in 1995
• In Kuma Hospital’s report on 1,235 people (2014), 8.0% had a tumor that grew by 3 mm or more over 10 years, and 3.8% developed new lymph node metastasis. In the later report on 3,222 people (2023), the figures are 4.7% and 1.0%. No one is reported to have died of thyroid cancer during or after observation
• Younger people tend to progress more (clinical progression in 10 years was 22.5% for those under 40 and 1.6% for those 60 and over, 2014 report)
• The Japan Thyroid Association also writes: “It is a serious mistake to say that a thyroid tumor of 1 cm or less can be left alone.” Surveillance is, in principle, continued for life under a specialist

Does food have an effect?

Iodine: “too little or too much”

Thyroid hormone is made from iodine. In regions with extreme iodine deficiency, goiter (an enlarged thyroid) and follicular cancer are common, and it is known that when iodine becomes sufficient, for example through the spread of iodized salt, the share of papillary cancer rises.

• Iodine intake of Japanese people is estimated at 1 to 3 mg per day on average (Dietary Reference Intakes for Japanese, 2025 edition, Ministry of Health, Labour and Welfare). In a 16-day diet record of 240 adults, about 60% of iodine came from kombu and about 30% from dashi (Katagiri 2015)
• In the Dietary Reference Intakes for Japanese (2025 edition), the adult recommended dietary allowance is 140 µg per day and the tolerable upper intake level is 3,000 µg (the upper limit for pregnant and breastfeeding women is 2,000 µg). The WHO recommended intake is 150 µg, and the upper limit in the U.S. and Canada is 1,100 µg. Japan’s upper limit is high because Japanese people get iodine from kombu, and iodine from kombu is thought to be poorly absorbed
• Too much iodine can make it harder for thyroid hormone to be made. The Dietary Reference Intakes include Japanese reports of hypothyroidism and goiter in people who took 28 mg a day from kombu dashi broth for about a year, and in people who ate one bag of kombu chips every day for about a month. People with Hashimoto’s disease in particular are said to be prone to hypothyroidism from too much iodine

Studies of Japanese people and seaweed

(All confirmed from abstracts)

• The Japanese and Korean studies give opposite directions. In a 2025 report summarizing environmental factors for thyroid cancer, 86% of the evidence was rated “very low certainty” (Peng 2025)
• The National Cancer Center Japan’s “Evaluation List of Cancer Risk and Preventive Factors” has no entry for thyroid cancer. The Cancer Information Service says, “The causes of thyroid cancer are not clearly known”
• In 2016, the International Agency for Research on Cancer (IARC) rated the link between having a lot of body fat (obesity) and thyroid cancer as having “sufficient evidence.” In a pooled analysis of 22 studies (about 2.1 million people), each 5-unit increase in BMI meant 1.06 times the risk (Kitahara 2016)
• Some observational studies also find less thyroid cancer among people who smoke or drink. However, tobacco and alcohol raise the risk of many other cancers, so the National Cancer Center Japan recommends not smoking and drinking in moderation. It does not mean “tobacco or alcohol can prevent thyroid cancer.” (For alcohol and cancer, see also the article on types of alcohol and cancer.)

Iodine in one serving

Most of Japanese people’s iodine comes from kombu. Calculating the amount per serving from the food composition tables gives the following.

• Kombu dashi gives more than twice the upper limit in one bowl of miso soup. Dashi made only from katsuobushi (dried bonito) contains almost no iodine
• The tolerable upper intake level is “the amount at which no adverse health effects are expected when taken habitually.” It does not mean that going over it for a single day is dangerous
• Iodine in hijiki drops greatly when it is soaked and boiled (dried hijiki 45,000 µg → boiled 960 µg per 100 g)

Are there “good” and “bad” foods for the thyroid?

No food has been recognized as one that “prevents thyroid cancer if you eat it.” Here is what public agencies and medical societies say.

To sum up, what the public sources support is not to look for a “good food” but to avoid stacking up kombu (dashi, tsukudani, kombu rolls and supplements). Managing body weight is one of the few factors that has evidence for thyroid cancer too.

Dashi for when you want to limit iodine

Before radioactive iodine treatment (isotope treatment), or when your doctor tells you to limit kombu, choosing a dashi whose ingredient list (原材料名) does not contain “昆布” (kombu), “こんぶエキス” (kombu extract) or “海藻” (seaweed) can greatly reduce iodine. Many “dashi no moto” (dashi powder) and “tsuyu” (noodle broth) products contain kombu or kombu extract, so check the back label. (For how to read umami ingredients, see also the article on umami seasonings.)

You can also simmer shaved dried fish instead of using dashi packs. “だしの力” (Dashi no Chikara) from Marutomo, a katsuobushi maker, is shaved dried fish for making dashi, and its only ingredients are fish fushi (dried fish).

• Fish dashi also contains a little iodine, but on a completely different order of magnitude from kombu (Figure 4)
• The length of the iodine restriction and how far to cut back depend on the treatment. Follow the diet instructions given by your hospital

Does radiation exposure have an effect?

It is established that exposure in childhood increases it

• The thyroid takes up iodine to make hormone, so radioactive iodine released in a nuclear accident (iodine-131) also collects in the thyroid (Ministry of the Environment)
• Among atomic bomb survivors (the Radiation Effects Research Foundation’s follow-up of about 105,000 people), the excess relative risk at age 60 for people exposed at age 10 was 1.28 per gray (1,000 mGy). The risk falls the older the person was at exposure, and for exposure at age 20 or later there is almost no evidence of an increase (Furukawa 2013). The Nuclear Regulation Authority also says “no statistically significant increase in risk of thyroid cancer has been confirmed in people exposed in adulthood or later”
• A pooled analysis of studies of external exposure in childhood found increases in line with dose even at thyroid doses below 200 mGy, and no clear threshold (a line below which exposure is “safe”) (Lubin 2017)

Chernobyl

• Among people who were under 18 at the time of the 1986 accident (all of Belarus and Ukraine, and four contaminated regions of Russia), about 20,000 thyroid cancers were registered from 1991 to 2015 (UNSCEAR 2018)
• The share estimated to be caused by radiation is about 25% (range 7–50%); the rest is attributed to the natural rise with age, increased attention, and improved diagnostic technology
• Children’s exposure was caused mostly by drinking milk containing iodine-131 in the weeks after the accident. The average thyroid dose of evacuated children and adolescents was about 900 mGy, and for preschool children it was 2 to 4 times that
• One study found radiation risk about 3 times higher in iodine-deficient areas, and about one-third as high in people who took stable iodine tablets (Cardis 2005)
• Thyroid cancer began to increase 4 to 5 years after the accident (Ministry of the Environment)

Fukushima

• UNSCEAR estimates that the thyroid dose to a 1-year-old in Fukushima in the first year after the accident was at most about 30 mGy in evacuated municipalities and at most about 20 mGy in areas not evacuated. That is far lower than the average for evacuated children at Chernobyl (about 900 mGy)
• Fukushima Prefecture continues to give thyroid ultrasound examinations to all residents who were roughly 18 or younger at the time of the accident. The numbers of people found “malignant or suspected malignant” by cytology are 116 in the 1st round, 71 in the 2nd, 31 in the 3rd, 39 in the 4th, 50 in the 5th, 24 in the 6th, 3 in the 7th, and a total of 37 in the examinations at ages 25 and 30 (data for the 6th round onward and the milestone examinations are from materials of the prefecture’s review committee as of March 31, 2026; rounds 1–5 are from Ministry of the Environment materials)
• UNSCEAR concluded that the larger-than-expected number of thyroid cancers found is “not the result of radiation exposure” but is because ultra-sensitive screening found thyroid abnormalities that had not been known before. As grounds, it cites that at Chernobyl there was no increase for four years after the accident, whereas in Fukushima 116 were found in the first examination; that almost none were found in Fukushima among children exposed at ages 0–4, who were numerous at Chernobyl; and that when three non-exposed prefectures were examined with the same method, cysts and nodules were found at about the same rate
• The Fukushima Prefecture Thyroid Examination Evaluation Subcommittee (July 2025) also states, for rounds 1–5, that “a link between thyroid cancer and radiation exposure was not recognized”
• On the other hand, there are papers that claim a link with exposure (such as Tsuda 2016), and UNSCEAR also notes that both views exist
• An expert group of the International Agency for Research on Cancer (IARC) (2018) did not recommend thyroid screening of the whole population after a nuclear accident, and proposed that long-term follow-up be considered for people who received 100–500 mGy or more from the fetal period through adolescence

Radiation from medical tests

• In the Ministry of the Environment’s materials (fiscal 2025 edition), the effective dose of one chest X-ray (frontal) is 0.08 mSv, of an intraoral dental X-ray 0.006–0.024 mSv, and of a CT scan about 0.06–17 mSv. The Ministry of the Environment says, “The exposure dose at which health effects on people have been confirmed is 100 mSv or more”
• There are studies (Australia, South Korea) showing a link between CT scans in childhood and thyroid cancer, but they are observational studies using insurance data
• For dental X-rays and thyroid cancer, a pooled review rated the certainty of the evidence as “low” (Chadli 2026). The American Academy of Oral and Maxillofacial Radiology (2023) says the thyroid risk of today’s dental imaging is negligible and recommends not using thyroid shields
• If you are worried about whether a test is necessary and about the radiation, talk to your doctor before you have the test

Kombu and gargle solutions do not protect against radiation exposure

The Nuclear Regulation Authority says that taking iodine-containing drugs and foods in place of stable iodine tablets “is inappropriate.” The National Institute of Radiological Sciences (2011) urged people “absolutely not” to drink gargle solutions or disinfectants, and explained that seaweed such as wakame does not work because its iodine content is not constant and it takes time to be absorbed. Stable iodine tablets are to be taken following instructions from the national or local government.

What Thyradin is made of, and what taking it does to the body

The active ingredient is “synthetic T4”

The active ingredient of Thyradin-S (チラーヂンS; manufactured and marketed by Aska Pharmaceutical, sold by Takeda Pharmaceutical) is levothyroxine sodium hydrate. The package insert says it “shows the same pharmacological action as T4 secreted from the thyroid in the body,” and the interview form (drug information file) says it is “a synthetic product with consistent potency.”

In the past, “desiccated thyroid,” made by drying and powdering the thyroid glands of cattle, pigs and other animals, was used as a thyroid hormone drug. Aska Pharmaceutical’s “Thyradin powder” (desiccated thyroid) was discontinued in 2015 (Thyradin-S interview form). Today’s Thyradin-S is not made from animal thyroid glands.

Additives (from the package insert)

(PMDA package insert revised June 2024; interview form revised November 2024)

• No strength contains lactose
• Iron sesquioxide is a colorant used to color-code the tablets. Each strength has a different color to prevent mix-ups
• The origin of the raw materials of the additives (plant or animal) is not stated in the public materials. If you are concerned, contact the Aska Pharmaceutical drug information center (listed in the package insert)
• The generic drug with the same ingredient (Levothyroxine Na tablets “Sandoz”) has additives such as potato starch and crystalline cellulose, and it also contains no lactose. Thyronamin, a T3 drug, has a different active ingredient and is a drug that starts working quickly and lasts a short time

What taking it does to the body

Thyradin-S is a drug that supplements thyroid hormone that is lacking. When the dose is right, it works just like the T4 in your body. Effects appear mainly when the dose is too high.

In papers, the following has been reported for people whose TSH was kept so low it could not be measured:

• Among people aged 60 and over, those with TSH of 0.1 or lower had atrial fibrillation in 28% over 10 years (11% in people with normal TSH) (Sawin 1994)
• In a study of about 18,000 people taking thyroid hormone drugs, people whose TSH was suppressed to 0.03 or lower had about twice the fractures and 1.6 times the arrhythmia. In people with lower-than-usual TSH of 0.04–0.4, there was no increase (Flynn 2010)
• In a Japanese study, in the group with strongly suppressed TSH, bone mineral density of the lumbar spine fell, especially in women aged 50 and over (Sugitani 2011)

“Replacement,” which keeps TSH in the reference range, and “suppression,” which keeps it below the reference range, differ in both purpose and effect on the body.

How to take it, and interactions

• The package insert’s directions are “oral administration once a day,” with no specification of timing such as “on an empty stomach.” The American Thyroid Association guideline (2014) recommends taking it “60 minutes before breakfast” or “at bedtime (at least 3 hours after dinner),” under the same conditions every day
• The package insert’s “use with caution when combined” section lists iron preparations, aluminum-containing stomach medicines (antacids), calcium carbonate, cholestyramine and others, and says “leave as much interval as possible from this drug” (because they bind to the drug in the digestive tract and reduce absorption). Calcium and iron supplements need caution for the same reason
• Although it is not in the package insert, papers report that taking it with coffee reduced absorption by 27–36% (Benvenga 2008), and that drugs that suppress stomach acid (proton pump inhibitors) increased the dose needed (Centanni 2006). One review also concludes that soy “seems to have a small clinical effect” (Gatta 2023)
• Changing the time you take it or what you take it with can change your TSH. If you want to change it, talk to your doctor or pharmacist

Does taking Thyradin for a long time damage the kidneys or other organs?

People who had the whole thyroid removed, or whose hormone has become insufficient, keep taking Thyradin-S for years or decades. For the worry that “taking it for a long time puts a strain on the kidneys and liver,” we checked organ by organ against the package insert, Japanese and U.S. guidelines, and papers.

• Kidney disease is not listed among the side effects in the Thyradin-S package insert. The American Thyroid Association guideline (2014) says, “There is no need to adjust the dose of levothyroxine in cirrhosis or kidney failure”
• What becomes a problem with long-term use is mainly a state of too high a dose (TSH suppressed too low) that continues. The heart and bones are the most likely to be affected
• Conversely, a state of too low a dose also affects kidney test values and the heart
• Liver disorder can rarely occur regardless of dose (frequency unknown in the package insert)

Kidney: no report of damage found. Too little makes the test value worse

When thyroid hormone is insufficient, serum creatinine, a guide to kidney function, rises.

• In a study of 24 people who deliberately stopped their hormone drug for a thyroid cancer test, creatinine while off the drug averaged 1.17 mg/dL, higher than while on it (0.87 mg/dL), and went back to normal when the drug was restarted (Kreisman 1999)
• A Swiss study also found creatinine fell once replacement began (Goede 2009). In a Chinese study of 40 people, those whose kidney values were poor with hypothyroidism improved with replacement (Zhao 2026; a small study with no comparison group)
• Studies of people with chronic kidney disease and slightly high TSH (subclinical hypothyroidism) have split results. In a Korean study of 309 people, kidney function declined more slowly in those who were given replacement (Shin 2012). On the other hand, in a British study of about 150,000 people, replacement neither reduced nor increased kidney deterioration (dialysis, or eGFR falling by half or more) (Yu 2025, hazard ratio 0.97). A U.S. veterans study also found no difference in eGFR (Hennessey 2021; a study funded by the pharmaceutical company AbbVie, with company employees among the authors)

No study has found that levothyroxine worsened the kidneys. However, there is not enough evidence to say that it “protects” the kidneys either.

• Cystatin C, another kidney test, conversely reads low when thyroid hormone is lacking and rises with replacement (Goede 2009). Whether the movement of the numbers is due to the kidneys or the thyroid is judged by your doctor looking at both
• People with kidney disease who take drugs that lower phosphorus (lanthanum carbonate, sevelamer) or drugs that lower potassium (calcium polystyrene sulfonate, sodium polystyrene sulfonate) need to be careful. They are listed under the package insert’s “use with caution when combined” because they can reduce the absorption of Thyradin-S, and the advice is to “leave as much interval as possible”
• In nephrotic syndrome, in which large amounts of protein are lost in the urine, the proteins that carry thyroid hormone are also lost in the urine, so the required dose of Thyradin may increase (American Thyroid Association guideline, 2014)

Heart: too much causes atrial fibrillation and more. Too little is not good either

• Serious side effects in the package insert include angina and congestive heart failure, and both are described as “possible overdose.” For people who have had angina or a myocardial infarction, and for older people, the advice is to start with a small dose and increase it slowly
• As introduced above in “What taking it does to the body,” atrial fibrillation and arrhythmia have increased in people whose TSH was suppressed so low it could not be measured (Sawin 1994, Flynn 2010)
• In a U.S. veterans study of about 700,000 people, people with TSH below 0.1 (too much) had 1.39 times the risk of death from cardiovascular disease. On the other hand, people with TSH over 20 (too little) had 2.67 times (Evron 2022). The result is that both too much and too little are bad. In the British study of about 18,000 people (Flynn 2010) as well, people with high TSH (too little hormone) had about 1.8 to 2 times more cardiovascular disease, arrhythmia and fractures
• Among 518 people treated for thyroid cancer in the Netherlands, atrial fibrillation was about 2.5 times that of people matched for age and sex. However, in this study it was not related to TSH values, and it increased slightly with larger doses of radioactive iodine (Klein Hesselink 2015)

Bone: strong TSH suppression lowers bone density in postmenopausal women

• A pooled analysis of 17 studies of people who received TSH-suppressing treatment for thyroid cancer found that bone mineral density of the lumbar spine was lower in postmenopausal women. It was not lower in premenopausal women and men (Ku 2021)
• For ordinary replacement without TSH suppression, a pooled analysis of 13 studies found a “slight adverse effect” (in people with hypothyroidism) to “no difference” (in people with the subclinical form) (Li 2025)
• The Japanese Clinical Practice Guidelines for Thyroid Tumors (2024) lists osteoporosis and cardiovascular disease as adverse events of TSH-suppressing treatment, and says that keeping TSH so low it cannot be measured “should not be done, because it not only fails to improve prognosis but also induces adverse events”

Liver: rarely, liver injury occurs as a reaction to the drug itself

• The package insert lists “liver dysfunction, jaundice” among serious side effects (frequency unknown). In case reports, a reaction of the body to the drug, separate from too high a dose, is thought to be the cause
• In Japan, there is a report of a 63-year-old man who developed liver injury about 2 months after starting, improved when the drug was stopped, and had no problem after switching to a T3 drug (Kawakami 2007). A case in which Thyradin-S caused liver injury after a bone marrow transplant has also been reported (Okamura-Shiki 2019)
• All are reports of single patients (case reports), and how often this happens is not known. If you have fatigue, fever, or yellowing of the skin or the whites of the eyes, tell your doctor

Brain: in older people, over-suppression increased diagnoses of cognitive disorders

In a study of about 66,000 U.S. people aged 65 and over, people whose TSH was lowered by the drug were diagnosed with mild cognitive impairment or dementia 1.34 times as often (Adams 2023). Because it is an observational study, it has not been established that the drug is the cause, but the authors point out that it is common for TSH to become too low on thyroid hormone drugs.

Cancer: a “link” appeared in a Taiwanese study, but whether it is a cause is not known

In a Taiwanese study comparing about 600,000 cancer patients with about 2.4 million people, people who had been taking levothyroxine had 1.5 times the cancer overall (Wu 2021). However, the authors themselves list as limitations that it is a case-control study and so causation cannot be judged, and that the effect of unmeasured factors is unknown, and they say more research is needed. You cannot say from this one study that “Thyradin causes cancer.”

What matters when taking it for a long time

• Have your TSH (and FT4) measured regularly to confirm that the dose is right. The target TSH differs by person, depending on factors such as how large the risk of thyroid cancer recurrence is
• To avoid atrial fibrillation and osteoporosis from hormone excess, the American Thyroid Association guideline (2014) recommends avoiding a dose that brings TSH below 0.1, especially in older people and postmenopausal women (for people with a high risk of recurrence of thyroid cancer, the Japanese guideline also recommends TSH-suppressing treatment after total thyroidectomy. How far to suppress is decided by your doctor weighing benefits and harms)
• The dose you need can change with age. The American Thyroid Association guideline says older people often need a smaller dose because muscle and other tissue decrease
• Do not reduce or stop it on your own. Too little also affects kidney test values and the heart
• Palpitations, a fast pulse, tremor, insomnia, weight loss and heavy sweating are symptoms the package insert calls “possible overdose.” If they continue, talk to your doctor

If one side is removed, does a rising TSH make recurrence more likely?

A higher TSH after one-sided removal is common

When half of the thyroid (one lobe) is removed, the remaining thyroid alone may not make enough hormone, and TSH rises in some people (Figure 5). In a Japanese report, 54.2% of 306 people who had one side removed developed hypothyroidism after surgery, and 95% of those did so within a year. It tended to be more common in people whose TSH before surgery was on the high side (2.0 or above) (Takata 2025). The Cancer Information Service says, “If more than half of the thyroid remains, treatment is in many cases unnecessary.”

What the guidelines say

ATA 2025 explains that if the target is “within the reference range,” 70–80% of people who had one side removed do not need hormone medicine, whereas if it is set at 0.5–2, only 20–30% do not need it. From 2015 to 2025, the insistence on keeping TSH low has weakened.

What the studies found

(All confirmed from abstracts. The number of recurrences is small, and most are observational studies.)

• While many results say “keeping TSH low does not reduce recurrence,” there are also studies saying “recurrence rises slightly if TSH stays on the high side,” and it is not settled
• A randomized controlled trial limited to people who had one side removed (the Korean MASTER trial, comparing a TSH 0.3–1.99 group with a 2.0–7.99 group) is under way, but as of September 2026 the recurrence results have not yet been published
• What the studies agree on is that “there is no need to keep TSH lower than the reference range (in low- and intermediate-risk cases).” Whether to add hormone medicine when TSH goes above the reference range, ATA 2025 says to “start” it. What to do with your own TSH value is something to decide with your doctor, who knows your risk classification

How to think about a rising thyroglobulin

What thyroglobulin is

Thyroglobulin (Tg) is a protein made in the thyroid and is the precursor of thyroid hormone. A normal thyroid and differentiated cancer (papillary and follicular cancer) both make it, and the two cannot be told apart. It also rises with benign tumors and thyroid inflammation, so it cannot be used to diagnose cancer (Cancer Information Service).

In people who had the whole thyroid removed and the remainder eliminated with radioactive iodine, Tg should in theory be nearly zero, so if it rises, it is a clue to recurrence. This is why “Tg is a tumor marker for thyroid cancer” is said.

After one-sided removal, the situation is different

After one-sided removal, the remaining thyroid keeps making Tg. In addition, when TSH rises, the remaining thyroid is stimulated and Tg also increases (ATA 2025).

Japan and the U.S. have different approaches. ATA 2025 writes, “Tg after one-sided removal has not been shown to predict recurrence. However, it has not yet been studied enough.”

The studies also have divided results.

• In people who had one side removed, Tg rose slowly by about 10% a year even without recurrence, and there was no difference between people who had a recurrence and those who did not (Park 2018, 208 people)
• Over 2 years, Tg fell in 42% of people, stayed the same in 22%, and rose in 36%, and none of these predicted recurrence (Ritter 2020, 167 people)
• On the other hand, there are reports of more recurrence in people with a high first Tg after surgery (Xu 2021, 1,451 people), and more neck lymph node recurrence in people whose Tg rose to more than twice its lowest value (Cho 2022)
• A cutoff of “reassuring if below 30 ng/mL” is sometimes used, but it has not been validated in any study (Tourani 2021)

Reasons Tg rises or moves other than cancer

• TSH went up (the remaining thyroid is stimulated)
• There is a nodule (lump) in the remaining side, or the thyroid is enlarged
• Thyroid inflammation (Hashimoto’s disease, thyroiditis)
• After fine-needle aspiration cytology (a needle test). One report says it returns to normal in about 2 weeks (Luboshitzky 2006)
• Differences in the testing method. If anti-thyroglobulin antibody (TgAb) is present, Tg can read lower than it really is, so measuring TgAb together with Tg is the basic practice

The guidelines’ approach is not to react to the Tg number alone but to have your doctor judge it together with the TSH value, TgAb and ultrasound results.

Thyroid cancer tumor markers and CA19-9

The marker used differs by type

(From the Clinical Practice Guidelines for Thyroid Tumors 2024, the Cancer Information Service and ATA guidelines)

Does CA19-9 rise in thyroid cancer?

CA19-9 is a tumor marker used mainly for pancreatic cancer and bile duct cancer. It is not described as a marker for thyroid cancer in the Japanese Guideline 2024, the Cancer Information Service or ATA 2025.

However, it is not that it never rises in thyroid cancer.

• Advanced medullary cancer: In an Italian study, CA19-9 was high in 16% of 100 people with advanced medullary cancer, and was normal in every person who was cured. People with high CA19-9 had a higher risk of death (Elisei 2015). In a study of 107 people from the same group, a high CA19-9 was also associated with death (Lorusso 2021)
• Papillary cancer: In a Japanese study of 196 people (2 cm or larger), CA19-9 was high before surgery in 6.1%, and became normal in all after surgery (Kihara 2024). Other reports are mainly case reports

CA19-9 also rises for reasons other than cancer.

• Diabetes (especially when blood sugar is poorly controlled)
• Gallstones, bile duct disease, jaundice, liver disease, pancreatitis
• Lung diseases such as bronchiectasis and interstitial pneumonia
• In a Korean study of about 500 people whose CA19-9 was high at a health checkup, cancer was found in 2.8%, a benign disease in 27.5%, and no cause in 69.7% (Kim 2009)
• There is a case report of it rising with hypothyroidism, but in a small study there was no difference between people with Hashimoto’s disease and healthy people

In addition, people who are “Lewis-negative” in the Lewis blood group system (about 1 in 10 in a Chinese study) cannot make CA19-9, so it does not rise even when cancer is present.

If you have been told your CA19-9 is high, first talk with your doctor about possibilities such as the pancreas, bile ducts, liver and diabetes, rather than thyroid cancer.

Life after surgery

The scar on the neck

Thyroid surgery is done through the front of the neck, so some people worry about how noticeable the scar is. After the wound has closed, there is a method of using tape for scars to keep the skin from being pulled. Nichiban says of “Atofine” that a “scar” means “a state in which the wound is completely closed,” and it warns not to use it directly on an open wound or on stitches before they are removed. Ask your surgeon about when to start using it.

For people who want to understand the disease through diagrams

Who should be careful

• People with a lump in the neck, hoarseness or difficulty swallowing that continues: See a doctor. People with symptoms are a separate matter from the screening discussion
• People with a family member who has medullary cancer or MEN2: It may be hereditary. Consult a specialist
• People who had radiation therapy to the neck in childhood: The guideline also treats them as people with “special risk”
• People taking Thyradin: Do not change the dose yourself. When you start iron, stomach medicine, calcium or similar, tell your doctor or pharmacist. Having your TSH measured regularly to check the dose also helps protect the heart, bones and kidneys
• People with kidney disease who take drugs that lower phosphorus or potassium: They can reduce the absorption of Thyradin. Check the interval between doses with your doctor or pharmacist
• People who have been told to restrict iodine: Check the ingredient lists (原材料名) of dashi, tsuyu and kombu products, and follow the diet instructions from your hospital
• People who take kombu supplements or seaweed powder every day: The amount of iodine can easily become high, so if you have a thyroid disease, tell your doctor

Summary of the products featured in this article

	カネジョウ Katsuo Dashi Pack	キッチン大友 いいだし	マルトモ だしの力	ニチバン アトファイン	やさしいカラー図解 甲状腺の病気
Who it is for	People who want to limit kombu (iodine)	People who want a seafood dashi without kombu	People who want to make dashi from shaved dried fish	People who want to protect the neck scar after surgery	People who want to understand thyroid disease through diagrams
Contents	Katsuobushi only (no kombu or salt)	Mackerel fushi, round scad fushi, frigate mackerel fushi, dried boiled anchovies (no kombu or salt)	Fushi of frigate mackerel, mackerel, round scad and bonito (no kombu)	Scar care tape (SS–LL)	Illustrated book by the director of Ito Hospital (2024)
Price (tax included)	7 g × 16 bags, 1,990 yen (free shipping)	8 g × 20 bags, 3,132 yen (free shipping)	80 g × 4, 1,832 yen (shipping included)	From 1,458 yen per box	1,870 yen (free shipping)
Buy	See on Rakuten PR See on Amazon.co.jp PR See on Yahoo! Shopping PR	See on Rakuten PR See on Amazon.co.jp PR See on Yahoo! Shopping PR	かつお節・だしを楽しむ通販サイト【マルトモ公式直販】 PR	See on Rakuten PR See on Amazon.co.jp PR See on Yahoo! Shopping PR	See on Rakuten PR See on Amazon.co.jp PR See on Yahoo! Shopping PR

(Checked on September 30, 2026 on the sales pages of Rakuten Ichiba and the Marutomo official online shop. Prices and shipping fees may change.)

Urayomi’s view

• The “increase” in thyroid cancer is thought to be mainly because the technology and opportunities for finding it have increased. Mortality has not increased in Japan or worldwide
• As for food, both “avoiding iodine completely” and “taking a large amount” are bad for the thyroid. On a Japanese diet it is rare to run short of iodine, and if there is anything to watch out for, it is stacking up kombu
• TSH and thyroglobulin after one-sided removal are an area where even the Japanese and U.S. guidelines differ and no answer has been reached. It is important not to judge from the numbers alone, but to follow your course together with your doctor
• Be wary of information that states flatly “if you eat this you can prevent thyroid cancer” or “with this number it is a recurrence”

Sources

• National Cancer Center Japan, Cancer Information Service, “Thyroid Cancer” (updated August 2026), booklet “Thyroid Cancer” (created August 27, 2026), “Cancer Statistics by Cancer Type: Thyroid” (updated July 2026), tables of cancer statistics (incidence 1975–2023, mortality 1958–2024), “Evaluation List of Cancer Risk and Preventive Factors” (version of May 26, 2026)
• Ministry of Health, Labour and Welfare, “2017 National Cancer Registry 5-Year Survival Report”
• Japan Association of Endocrine Surgery, “Clinical Practice Guidelines for Thyroid Tumors 2024” (Journal of the Japan Association of Endocrine Surgery, vol. 41, special issue; published on Minds)
• Japan Thyroid Association, “Our View on Non-Surgical Observation of Very Low-Risk Papillary Thyroid Cancer in Adults” (2020)
• Japanese Society of Laboratory Medicine, “On Harmonization of Thyroid-Stimulating Hormone (TSH) Values” (2020); Japanese Association of Clinical Laboratory Specialists, “Thyroid Tests” (2024)
• PMDA, Thyradin-S tablets and powder package insert (revised June 2024) and interview form (revised November 2024); package inserts for Levothyroxine Na tablets “Sandoz” and Thyronamin tablets
• Ministry of Health, Labour and Welfare, “Dietary Reference Intakes for Japanese (2025 edition)”; MEXT, “Standard Tables of Food Composition in Japan (8th Revision), 2023 Supplement” (updated March 27, 2026)
• Ministry of the Environment, “Unified Basic Materials on the Health Effects of Radiation (fiscal 2025 edition)”; Fukushima Prefecture, Fukushima Health Management Survey review committee (60th meeting, July 30, 2026) materials
• Nuclear Regulation Authority Secretariat, “On the Distribution and Taking of Stable Iodine Tablets” (revised July 2019); National Institute of Radiological Sciences, “Do Not Drink Disinfectants Containing Iodine” (2011)
• UNSCEAR 2018 White Paper (Chernobyl), UNSCEAR 2020/2021 Report Annex B (Fukushima), UNSCEAR 2024 Annex B
• IARC GLOBOCAN 2024 (Cancer Today, version of July 8, 2026), Cancer Over Time, press releases No. 246 (2016) and No. 284 (2020)
• Ito Hospital, “Iodine and the Thyroid”; Kuma Hospital Thyroid Navi (Hashimoto’s disease and diet, iodine restriction); Noguchi Hospital, “The Thyroid and Iodine”; British Dietetic Association, “Iodine”; German Federal Institute for Risk Assessment (BfR), “Iodine in Dried Seaweed”
• Haugen BR, et al. Thyroid. 2016 (ATA 2015 guideline) / Ringel MD, et al. Thyroid. 2025 (ATA 2025 guideline) / Jonklaas J, et al. Thyroid. 2014 (ATA hypothyroidism guideline) / Wells SA, et al. Thyroid. 2015 (ATA medullary cancer guideline)
• USPSTF. JAMA. 2017 (thyroid cancer screening)
• Sugitani I, et al. J Clin Endocrinol Metab. 2010 / Sugitani I, et al. Surgery. 2011 (trials of TSH suppression and bone density)
• Lee MC, et al. Endocrinol Metab. 2019 / Park S, et al. Thyroid. 2017 / Park JH, et al. J Surg Oncol. 2018 / Xu S, et al. Thyroid. 2022 / Jeon YW, et al. Cancers. 2024 / Qiang JK, et al. Thyroid. 2025 / Won HR, et al. Cancers. 2022 / Lee EK, et al. Endocrinol Metab. 2021 (design of the MASTER trial) / Takata K, et al. Auris Nasus Larynx. 2025 (TSH and recurrence or reduced function after one-sided removal)
• Park S, et al. Thyroid. 2018 / Ritter A, et al. J Clin Endocrinol Metab. 2020 / Xu S, et al. Thyroid. 2021 / Cho JS, et al. Anticancer Res. 2022 / Tourani SS, et al. ANZ J Surg. 2021 / Luboshitzky R, et al. Endocr Pract. 2006 (thyroglobulin after one-sided removal)
• Elisei R, et al. Eur J Endocrinol. 2015 / Lorusso L, et al. Thyroid. 2021 / Kihara M, et al. JMA J. 2024 / Kim BJ, et al. Dig Liver Dis. 2009 / Kodama T, et al. J Clin Lab Anal. 2007 / Uygur-Bayramicli O, et al. World J Gastroenterol. 2007 / Scarà S, et al. Adv Exp Med Biol. 2015 / Guo M, et al. FEBS Open Bio. 2017 (CA19-9)
• Sawin CT, et al. N Engl J Med. 1994 / Flynn RW, et al. J Clin Endocrinol Metab. 2010 (low TSH, atrial fibrillation and fractures)
• Kreisman SH, Hennessey JV. Arch Intern Med. 1999 / Goede DL, et al. Swiss Med Wkly. 2009 / Zhao T, et al. Front Endocrinol. 2026 / Shin DH, et al. J Clin Endocrinol Metab. 2012 / Yu OHY, et al. CMAJ. 2025 / Hennessey JV, et al. Adv Ther. 2021 (thyroid hormone and the kidney)
• Evron JM, et al. JAMA Netw Open. 2022 / Klein Hesselink EN, et al. J Clin Endocrinol Metab. 2015 (heart) / Ku EJ, et al. J Clin Endocrinol Metab. 2021 / Li X, et al. BMC Endocr Disord. 2025 (bone)
• Kawakami T, et al. Intern Med. 2007 / Okamura-Shiki I, et al. Leuk Lymphoma. 2019 (case reports of liver injury) / Adams R, et al. JAMA Intern Med. 2023 (cognitive function) / Wu CC, et al. Cancer Sci. 2021 (cancer)
• Benvenga S, et al. Thyroid. 2008 / Centanni M, et al. N Engl J Med. 2006 / Gatta E, et al. Endocr Pract. 2023 (Thyradin absorption and food, drink and drugs)
• Ahn HS, et al. N Engl J Med. 2014 / Park S, et al. BMJ. 2016 / Lee M, et al. Thyroid. 2020 (South Korea) / Furuya-Kanamori L, et al. J Clin Oncol. 2016 (autopsy)
• Ito Y, et al. Thyroid. 2014 / Miyauchi A, et al. Thyroid. 2023 (active surveillance)
• Michikawa T, et al. Eur J Cancer Prev. 2012 / Kolonel LN, et al. Cancer Causes Control. 1990 / Kwon YJ, et al. Nutrients. 2024 / Cao LZ, et al. Medicine. 2017 / Cho YA, Kim J. Nutr Cancer. 2015 / Peng G, et al. Endocrine. 2025 / Katagiri R, et al. Br J Nutr. 2015 (seaweed and iodine)
• Lauby-Secretan B, et al. N Engl J Med. 2016 / Kitahara CM, et al. Thyroid. 2016 (body fat)
• Furukawa K, et al. Int J Cancer. 2013 / Lubin JH, et al. J Clin Endocrinol Metab. 2017 / Cardis E, et al. J Natl Cancer Inst. 2005 / Tsuda T, et al. Epidemiology. 2016 (radiation)
• Chadli K, et al. BMC Oral Health. 2026 / Benavides E, et al. J Am Dent Assoc. 2023 (dental X-rays)
• Nichiban, “Atofine” product page (precautions for use)