- 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 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 vegans who might become pregnant 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 [NIH, 2020]. These components, called goitrogens, can cause goiter. Thus, large amounts of soy combined with inadequate iodine intake can exacerbate iodine deficiency.
Theoretically, consuming large amounts of goitrogens could have a negative impact even when iodine intake is adequate; however, we haven’t found evidence of this for people who aren’t exposed to unusually large amounts, such as people whose diets are largely based on cassava root. It might only be a concern in areas of the world where goiter is prevalent, such as parts of India [Chandra Amar, 2015; LPI, 2015]. As long as vegans maintain adequate iodine intake, they shouldn’t need to worry about goitrogens [NIH, 2020].
Iodine is consistently found in only a few foods such as dairy products (cows are given iodine supplements and iodine solutions are used to clean the cows’ teats and dairy equipment and end up in the milk) and sea animals and seaweeds [NIH, 2020].
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. Commercial plant milks are very low in iodine [Ma, 2016; Bath, 2017] . Plant milks fortified with iodine have an iodine content similar to cow’s milk but only a few products are iodine-fortified [Bath, 2017; Vance, 2017].
In the United States, 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 usually contain iodine, but iodized sea salt is available for purchase.
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 [Amster, 2007].
A survey of kelp supplements in the U.S. found that eight out of nine batches contained some level of arsenic [Amster, 2007]. Another survey in the UK of imported seaweed found very little arsenic in kelp [Rose, 2007].
A 2017 ConsumerLabs.com report found arsenic contamination in one of six supplements they tested in the U.S. However, they found too much iodine in many of the supplements and between this and the potential for arsenic contamination, it’s probably best to opt for a potassium-iodine supplement that isn’t from kelp.
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 [Leung, 2011]. 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 varying amounts of iodine, and salt iodization is not mandated in the majority of European countries [EUFIC, 2019]. Some areas have other iodization programs, such as iodized oil in Romania and iodized water in Sicily [EUFIC, 2019]. Studies have shown that vegans in Europe who do not supplement (as well as those who oversupplement) have indications of abnormal thyroid function [Key, 1992; Lighthowler, 1998] and inadequate (or excessive) iodine intake [Brantsaeter, 2018].
A 2020 systematic review summarized findings of 15 research articles investigating iodine status of omnivores, vegetarians, and vegans in Europe and the U.S. (the U.S. study is the 2011 Boston study mentioned above) [Eveleigh, 2020]. They found vegetarian and vegan median urinary iodine concentrations fell below national values in their respective countries in all studies except for one (in Finland, where national values were already insufficient). Only three studies reported iodine supplement use in vegans, which contributed between 9.0 to 54.0 μg iodine/day. Although some vegans had excessive iodine intake due to seaweed, overall, vegans were at the highest risk for low iodine status.
Please see our list of Daily Needs for how to obtain adequate Iodine. Avoid iodine intakes in excess of the Upper Limit.
Last updated July 2020
Amster, 2007. 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.
Bath, 2017. Bath SC, Hill S, Infante HG, Elghul S, Nezianya CJ, Rayman MP. Iodine concentration of milk-alternative drinks available in the UK in comparison with cows’ milk. Br J Nutr. 2017 Oct;118(7):525-532.
Brantsaeter, 2018. 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).
Key, 1992. 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.
Lighthowler, 1998. Lightowler HJ, Davies GJ. Iodine intake and iodine deficiency in vegans as assessed by the duplicate-portion technique and urinary iodine excretion. Br J Nutr 1998 Dec;80(6):529-35.