Iodine

Is iodine safe?

At the dose most people take, yes. A standard supplement gives 150 mcg, usually as potassium iodide, which is the US daily recommendation and far below the upper limit of 1,100 mcg a day in the US or 600 mcg in the EU. During pregnancy and breastfeeding it is actively recommended. The problems start with kelp and seaweed products, which can hold many times the upper limit in a single tablet, and with high-dose "iodine protocols" spread online. People with thyroid disease are the most at risk.

How much iodine is too much?

The two official upper limits differ. The US National Academy of Medicine allows 1,100 mcg a day for adults; the EU limit, set by the Scientific Committee on Food in 2003 and still used by EFSA, is 600 mcg for adults, including during pregnancy and breastfeeding. Both cover intake from food and supplements together.

In healthy people with enough iodine, raised TSH, the first sign that the thyroid is struggling, has been seen at about 1,700 mcg a day or more in adults, according to the Linus Pauling Institute. The American Thyroid Association advises against taking 500 mcg a day or more from iodine, potassium iodide or kelp supplements.

Those limits do not protect everyone. People with nodular goitre, Hashimoto's thyroiditis, Graves' disease or part of their thyroid removed can react at lower intakes, as can older people, newborns and pregnant women.

How excess iodine upsets the thyroid

The thyroid has a safety brake. When a large dose of iodine arrives, it briefly stops making hormone, the Wolff–Chaikoff effect, and then normally escapes from that block after a short time. A 2026 review by UK endocrinologists explains that if the thyroid fails to escape, the result is hypothyroidism and sometimes goitre. In a thyroid with autonomous nodules or Graves' disease, the opposite can happen: the extra iodine fuels too much hormone, called the Jod-Basedow effect.

Case reports show both. A 39-year-old woman with goitre became hyperthyroid after four weeks of a kelp tea from an alternative practitioner (2006). A 45-year-old woman with no thyroid history had two months of hyperthyroidism after a kelp-containing diet product, then an underactive thyroid that needed levothyroxine for three months (2014). In Australia, a soy milk made with kombu seaweed caused thyroid problems in adults and in a newborn whose mother drank it (2010).

Why kelp is the problem

Brown seaweeds such as kelp and kombu concentrate iodine from seawater, and the amount varies widely by species and product. The 2026 UK review gives typical ranges:

A 2021 Norwegian study measured 96 seaweed products. Supplements gave between 5 and 5,600 mcg per daily dose, and one portion of 54 of the products would exceed the upper limit. Labels for species and iodine content were inadequate or wrong for several of them. Our sea moss page covers the same problem in sea moss blends.

Potassium iodide, by contrast, comes as a fixed, labelled dose. The catch there is different: a 2022 label survey of supplements sold in Switzerland found that 7 of 23 prenatal multivitamins contained no iodine at all, so check the label rather than assume.

Pregnancy: too little matters as much as too much

Iodine needs rise to 220 mcg a day in pregnancy and 290 mcg when breastfeeding, because the baby depends entirely on the mother's supply for thyroid hormone and brain development. Mild deficiency has returned in the US: in national data for 2007 to 2014, 42% of non-pregnant women aged 15 to 44 had urine iodine below 100 mcg/L, a sign of low intake.

That is why the American Thyroid Association recommends 150 mcg a day from a supplement during pregnancy and breastfeeding. UK endocrinologists add that kelp should not be the source, because of both excess and unreliable labels. Germany's BfR proposed in December 2025 that supplements carry no more than 100 mcg per daily dose, or 150 mcg for pregnancy.

Medicines that interact with iodine

The Linus Pauling Institute lists amiodarone, a heart rhythm drug that itself contains a lot of iodine, antithyroid drugs such as methimazole, carbimazole and propylthiouracil, and lithium, all of which can add to thyroid effects. Drug doses of potassium iodide may also weaken warfarin. If you take levothyroxine, tell your doctor before adding iodine.

How to take it safely

1. Choose potassium iodide with the dose printed in mcg, not kelp or "seaweed extract".
2. Add up all sources: a multivitamin, a prenatal vitamin and a separate iodine tablet can stack quickly.
3. Stay under 500 mcg a day from supplements unless a doctor prescribes more.
4. Ask your doctor first if you have any thyroid condition, take amiodarone or lithium, or are pregnant and unsure what your prenatal contains.

How we scored it

Regulatory actions (78) reflect a legal nutrient with clear upper limits and a proposed German cap. Interactions (70) and dose safety (68) are held back by kelp products far above the limit and by thyroid sensitivity in vulnerable groups. Product quality (55) and organ toxicity (60) are the weakest, with unpredictable kelp labels and repeated thyroid case reports.