Iodine Deficiency & Thyroid Function

Iodine is needed for healthy thyroid function which regulates metabolism. Both too much, and too little iodine can result in abnormal thyroid metabolism. The UK Vegan Society provides an overview on idoine deficiency. An excerpt:

...Iodine deficiency during pregnancy and early infancy can result in cretinism (irreversible mental retardation and severe motor impairments).... Hypothyroidism can manifest as low energy levels, dry or scaly or yellowish skin, tingling and numbness in extremities, weight gain, forgetfulness, personality changes, depression, anaemia, and prolonged and heavy periods in women.... Hypothyroidism can also cause carpal tunnel syndrome and Raynaud's phenomenon [episodes of blood flow loss to the fingers, toes, ears, and nose]. Hypothyroidism can lead to significant increases in cholesterol levels and homocysteine levels and is implicated in about 10% of cases of high cholesterol levels. Correcting hypothyroidism can lead to a 30% drop in cholesterol and homocysteine levels.

According to Mayo Clinic, an overactive or underactive thyroid gland can also lead to hair loss.

Iodine Antagonists

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 Status of Vegetarians

Iodine is only found inconsistently in plant foods, depending on the iodine content of the soil. Food grown near the ocean tends to be higher in iodine. 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 a 2011 cross-sectional study from the Boston area, urinary iodine levels of 78 lacto-ovo vegetarians and 62 vegans were measured (4). People with previously diagnosed thyroid problems were excluded from the study. 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. Most of the vegans were making no effort to insure adequate iodine intake.

Iodine deficiency is not as much of a problem for U.S. vegans as it is for European vegans (1), whose food supply contains less iodine. Studies have shown that vegans in Europe (where salt is minimally iodized) who do not supplement (as well as those who oversupplement) have indications of abnormal thyroid function (1, 2).

Table 1. Dietary Reference Intake (DRI) for Iodine
Age (yrs) DRI (µg)a Upper Limitb (µg)
0 - 6 mos 110  
7 - 12 mos 130  
1 - 3 90 200
4 - 8 90 300
9 - 13 120 600
14-18 150 900
> 18 150 1100
≤ 18 220 900
> 18 220 1100
≤ 18 290 900
> 18 290 1100
aµg = microgram = mcg. | bDo not exceed the upper limit.

Recommendations and Sources

North American vegans should take a modest iodine supplement; 75-150 µg every few days.

Vegan iodine supplements can be found in most grocery or natural food stores, and most multivitamins contain iodine.

In the United States, you can get the extra 75 µg of iodine from 1/4 teaspoon of iodized salt. If you are already eating 1/4 teaspoon of salt per day on your foods, make sure it is iodized.

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.

There may be a connection between excess iodine and acne (5, 6), although amounts up to 1,000 µg are considered safe for the majority of population (7).

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 (8). A survey of kelp supplements in the U.S. found that eight out of nine batches contained some level of arsenic (8). Another survey in the UK of imported seaweed found very little arsenic in kelp, and no detectible amounts of inorganic arsenic, which is the harmful type (9).

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, you may be able to find a non-kelp source of potassium iodide.

For more info, see Arsenic in Kelp Supplements.

Avoid iodine intakes in excess of the Upper Limit.

Iodine Factoids



1. 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. Link. 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. Link

2. 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):529-35I. Link

4. Leung AM, Lamar A, He X, Braverman LE, Pearce EN. Iodine Status and Thyroid Function of Boston-Area Vegetarians and Vegans. J Clin Endocrinol Metab. 2011 May 25. [Epub ahead of print] Link

5. Arbesman H. Dairy and acne--the iodine connection. J Am Acad Dermatol. 2005 Dec;53(6):1102.   |   Link

6. Danby FW. Acne and iodine: reply. J Am Acad Dermatol. 2007 Jan;56(1):164-5.   |   Link

7. Pennington JA. A review of iodine toxicity reports. J Am Diet Assoc. 1990 Nov;90(11):1571-81. Review.   |   Link (Abstract)

8. 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.   |   Link | Follow-up letters to the editor.

9. Rose M, Lewis J, Langford N, Baxter M, Origgi S, Barber M, MacBain H, Thomas K. Arsenic in seaweed--forms, concentration and dietary exposure. Food Chem Toxicol. 2007 Jul;45(7):1263-7.   |   Link