Quick answer
NIH says high iodine intake can cause an overactive thyroid, and can also cause the opposite problems, an underactive thyroid and goiter. The adult upper limit is 1,100 mcg a day. Ask your doctor before taking iodine if you have a thyroid condition or take thyroid medicine.1,2
Key facts
- NIH lists an overactive thyroid among the effects of high iodine intake, along with goiter, raised TSH and an underactive thyroid.1
- NIH says iodine-induced hyperthyroidism can also follow iodine given to correct a shortfall.1
- People with autoimmune thyroid disease or a past iodine shortfall may react at intakes that are fine for most people.1
- The prescription potassium iodide label says iodides should be used with caution in people with hyperthyroidism, and that iodide can change thyroid tests.3
- Kelp supplements vary widely: in 96 products, 54 would pass the upper limit in a single dose.4
What does NIH say about iodine and an overactive thyroid?
High iodine intakes can cause some of the same problems as too little, including goiter, raised TSH and an underactive thyroid, and can also cause an overactive thyroid, thyroid inflammation and thyroid papillary cancer.1,2
NIH adds that iodine-induced hyperthyroidism can result from high intakes, including when iodine is given to correct a shortfall. It says responses to excess iodine, and the doses that cause trouble, vary from person to person.1
People with autoimmune thyroid disease or with a history of iodine deficiency may have problems at intakes that are safe for most people.1
How much iodine is too much?
The upper limit for iodine from all sources is 1,100 mcg a day for adults and 900 mcg for teens aged 14 to 18. It does not apply to people taking iodine as a treatment under a doctor's care.1,2
NIH says that for most people, iodine from food and supplements together is unlikely to pass the upper limit. The exceptions are concentrated products. Kelp and seaweed supplements are the clearest case: in a Norwegian analysis, daily doses ranged from 5 to 5,600 mcg, labels were often wrong and 54 of 96 products would exceed the upper limit in one portion or dose.1,4
Acute poisoning is rare and usually involves doses of many grams. Its signs include burning of the mouth, throat and stomach, fever, abdominal pain, vomiting and a weak pulse.1
Can iodine cause an underactive thyroid too?
Yes, in some people. NIH explains that excess iodine in susceptible individuals can block thyroid hormone production, which raises TSH and can enlarge the gland. That is why the same high intake can push thyroid tests in either direction.1
The prescription potassium iodide label says prolonged use of iodides can lead to an underactive thyroid, lists goiter and thyroid adenoma among possible side effects, and says thyroid function tests may be altered by iodide.3
What did trials in pregnancy find?
In a Cochrane review of iodine around pregnancy, the chance of an overactive thyroid during pregnancy was not clearly different with iodine (one trial of 365 women). Iodine went with a lower chance of an overactive thyroid after the birth, in three trials of 543 women. Both findings are low-quality evidence from places with mild to moderate iodine shortage.5
Who should ask a doctor about iodine and the thyroid?
- Anyone with a thyroid condition, including autoimmune thyroid disease, or who takes thyroid medicine.1
- Anyone who takes an antithyroid medicine or lithium, which the labels say can add to iodide's effect on the thyroid.3
- Anyone thinking about kelp, seaweed or an iodine-only supplement.4
- Anyone with a thyroid blood test or a radioactive iodine procedure coming up, because iodine intake can change results and preparation.1,3
Ask your doctor, and bring the Supplement Facts panel of every product that lists iodine, since multivitamins often contain it.
Questions people ask
Does iodine cause hyper or hypothyroidism?
It can cause either, according to NIH: high intake can produce goiter, raised TSH and an underactive thyroid, and can also produce an overactive thyroid. Ask your doctor if you have a thyroid condition.1
Does too much iodine cause hyperthyroidism?
NIH lists an overactive thyroid among the effects of high iodine intake. The adult upper limit it gives is 1,100 mcg a day from all sources, and it does not apply to iodine taken as treatment under a doctor's care.1
Can kelp supplements cause an overactive thyroid?
The sources checked do not link kelp to it by name. They show kelp supplements can hold very different amounts, from 5 to 5,600 mcg per daily dose in one analysis, so ask your doctor before using one.1,4
Is iodine safe with an overactive thyroid?
The potassium iodide label says iodides should be used with caution in people with hyperthyroidism. Ask your doctor before taking iodine, and tell them about any thyroid medicine.3
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Sources
- Office of Dietary Supplements, National Institutes of Health. Iodine: Fact Sheet for Health Professionals. Updated November 5, 2024. Opened 2026-10-07.
- Office of Dietary Supplements, National Institutes of Health. Iodine: Fact Sheet for Consumers. Updated May 1, 2024. Opened 2026-10-07.
- Potassium iodide oral solution (saturated): prescribing information (Emerald Therapeutics). DailyMed, U.S. National Library of Medicine. Effective August 25, 2026. Opened 2026-10-07.
- Aakre I, Solli DD, Markhus MW, et al. Commercially available kelp and seaweed products: valuable iodine source or risk of excess intake? Food Nutr Res. 2021;65:7584. Opened 2026-10-07.
- Harding KB, Peña-Rosas JP, Webster AC, et al. Iodine supplementation for women during the preconception, pregnancy and postpartum period. Cochrane Database Syst Rev. 2017;3:CD011761. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.