by Jack Norris, RD • Last updated October 2017
- Iodine Deficiency and Excess
- Iodine Sources
- Iodine Status of U.S. Vegetarians and Vegans
- Iodine Status of European Vegans
- Daily Needs
For the purposes of this article, iodine refers both to iodide (one atom) and iodine (a compound of two iodide atoms).
Iodine Deficiency and Excess
Iodine is needed for healthy thyroid function which regulates metabolism. Both too little—and too much—iodine can result in abnormal thyroid metabolism.
A goiter (enlarged thyroid gland) can be caused by eating both too little or too much iodine. The symptoms of either can be hypothyroidism—in which metabolism slows and weight and cholesterol increases—or hyperthyroidism—where metabolism increases resulting in weight loss.
An iodine deficiency can inhibit brain development in a fetus, so vegan women should ensure a reliable source of iodine.
There are components in soy, flax seeds, and raw cruciferous vegetables (broccoli, brussels sprouts, cauliflower, and cabbage) that counteract iodine. These components, called goitrogens, cause an enlarged thyroid gland, also called a goiter. Thus, large amounts of soy combined with inadequate iodine intake can exacerbate iodine deficiency.
Iodine is consistently found in only a few foods such as dairy products (iodine solutions are used to clean the cows’ teats and dairy equipment and end up in the milk) and seafood (including seaweed).
In plant foods, iodine is found inconsistently and depends on the iodine content of the soil—food grown near the ocean tends to be higher in iodine.
In the United State, iodine is added to iodized salt at a rate of 76 µg per 1/4 teaspoon (1.5 gram) of salt. This amount of salt also provides 580 mg of sodium.
If salt is iodized, it will say so on the package. The salt found in packaged foods is usually not iodized.
Sea salt doesn’t necessarily contain iodine.
Iodine and Seaweeds
If you regularly eat seaweed (multiple times a week), you will probably get adequate iodine from the seaweed. However, the availability of iodine from seaweed is variable and it can provide too much iodine. Cases of iodine toxicity reported in scientific journals are often from excessive amounts of kelp and kelp tablets.
Most iodine supplements are simply tablets made from kelp. Being a seaweed, kelp likely contains at least small amounts of arsenic. There are some very rare cases in which people taking kelp supplements have developed symptoms of arsenic toxicity (4).
A survey of kelp supplements in the U.S. found that eight out of nine batches contained some level of arsenic (4). Another survey in the UK of imported seaweed found very little arsenic in kelp, and no detectable amounts of inorganic arsenic, which is the harmful type (5).
It is very unlikely that, taken at recommended amounts of 150 µg every other day, arsenic toxicity is likely to occur from kelp supplements. However, if you are concerned, non-kelp potassium iodide supplements are rare but available.
For more info, see Arsenic in Kelp Supplements.
Iodine Status of U.S. Vegetarians and Vegans
In a 2011 cross-sectional study from the Boston area, urinary iodine levels of 78 lacto-ovo vegetarians and 62 vegans were measured (6). Most of the vegans were making no effort to ensure adequate iodine intake.
According to the authors, “Population iodine sufficiency is defined by median urinary iodine concentrations 100 µg/l or greater in adults and 150 µg/l or greater in pregnancy.”
Median urinary iodine concentration of vegans (79 µg/l; range 7 – 965 µg/l) was significantly lower than vegetarians (147 µg/l; range 9 – 779 µg/l). Markers of thyroid function were similar in both groups and in the normal range; one vegan and no vegetarians had abnormal thyroid function.
Iodine Status of European Vegans
The European food supply contains less iodine than the U.S., and salt is only minimally iodized. Studies have shown that vegans in Europe who do not supplement (as well as those who oversupplement) have indications of abnormal thyroid function (7, 8) and inadequate (or excessive) iodine intake (9).
Please see our list of Daily Needs for how to obtain adequate Iodine. Avoid iodine intakes in excess of the Upper Limit.
- American Thyroid Association – Iodine Deficiency
4. Amster E, Tiwary A, Schenker MB. Case report: potential arsenic toxicosis secondary to herbal kelp supplement. Environ Health Perspect. 2007 Apr;115(4):606-8. Epub 2007 Jan 18. Follow-up letters to the editor.
7. Key, T. J. A., Thorogood, M., Keenan, J. and Long, A. (1992), Raised thyroid stimulating hormone associated with kelp intake in British vegan men. Journal of Human Nutrition and Dietetics, 5: 323–326. Also described in:
Appleby PN, Thorogood M, Mann JI, Key TJ. The Oxford vegetarian study: an overview. Am J Clin Nutr 1999 Sep;70(3 Suppl):525S-531S.
9. Brantsaeter AL, Knutsen HK, Johansen NC, et al. Inadequate iodine intake in population groups defined by age, life stage and vegetarian dietary practice in a Norwegian convenience sample. Nutrients. 2018 Feb 17;10(2).