The relationship between iodine and thyroid function is complex. Iodine is required by the body to form thyroid hormone, and iodine deficiency can lead to goiter and hypothyroidism.¹ Severe and prolonged iodine deficiency can potentially lead to serious types of hypothyroidism, such as myxedema or cretinism. It is estimated that one and a half billion people living in 118 countries around the world are at risk for developing iodine deficiency.²

Today, most cases of iodine deficiency occur in developing nations. In industrialized countries where iodized salt is used, iodine deficiency has become extremely rare. On the other hand, iodine toxicity has become a concern in some of these countries.³ Excessive iodine intake can result in either hypothyroidism⁴ or hyperthyroidism (overactive thyroid).⁵ Sources of iodine include foods (iodized salt, milk, water, seaweed, ground beef), dietary supplements (multiple vitamin-mineral formulas, seaweed extracts), drugs (potassium iodide, amiodarone, topical antiseptics), and iodine-containing solutions used in certain laboratory tests. Many nutritional supplements contain 150 mcg of iodine. While that amount of iodine should prevent a deficiency, it is not clear whether supplementing with iodine is necessary or desirable for most people. Those wishing to take a nutritional supplement containing iodine should consult a doctor.

Bladderwrack (Fucus vesiculosus) is a type of brown seaweed that contains variable amounts of iodine.⁶ Hypothyroidism due to insufficient intake of iodine may possibly improve with bladderwrack supplementation, though human studies have not confirmed this.

Selenium plays a role in thyroid hormone metabolism. Severe selenium deficiency has been implicated as a possible cause of goiter.⁷ Two months of selenium supplementation in people who were deficient in both selenium and iodine was shown to induce a dramatic fall of the already impaired thyroid function in clinically hypothyroid subjects.⁸ Researchers have suggested that people who are deficient in both selenium and iodine should not take selenium supplements without first receiving iodine or thyroid hormone supplementation.⁹ There is no research demonstrating that selenium supplementation helps people with hypothyroidism who are not selenium-deficient.

Desiccated thyroid, also called thyroid extract (e.g., Armour Thyroid), is used by some doctors as an alternative to synthetic thyroid hormones (such as thyroxine [Synthroid or other brand names]) for people with hypothyroidism. Thyroid extract contains two biologically active hormones (thyroxine and triiodothyronine), whereas the most commonly prescribed thyroid-hormone preparations contain only thyroxine. One study has shown that the combination of the two hormones contained in desiccated thyroid is more effective than thyroxine alone for those with hypothyroidism.¹⁰ One doctor reported that thyroid extract worked better than standard thyroid preparations for many of his patients with hypothyroidism.¹¹ Glandular thyroid products, which are available from health food stores, have had most of the thyroid hormone removed and would therefore not be expected to be effective for people with hypothyroidism. Intact desiccated thyroid is available only by prescription. Hypothyroidism sufferers who want to use desiccated thyroid must first consult with a physician.

People with hypothyroidism have been shown to have an impaired ability to convert beta-carotene to vitamin A.^12,13 For this reason, some doctors suggest taking supplemental vitamin A (approximately 5,000–10,000 IU per day) if they are not consuming adequate amounts in their diet.

Preliminary data indicate that vitamin B3 (niacin) supplementation may decrease thyroid hormone levels. In one small study, 2.6 grams of niacin per day helped lower blood fat levels.¹⁴ After a year or more, thyroid hormone levels had fallen significantly in each person, although none experienced symptoms of hypothyroidism. In another case report, thyroid hormone levels decreased in two people who were taking niacin for high cholesterol and triglycerides; one of these two was diagnosed with hypothyroidism.¹⁵ When the niacin was discontinued for one month, thyroid hormone levels returned to normal.

Laboratory animals with severe, experimentally induced zinc deficiency developed hypothyroidism, whereas moderate zinc deficiency did not affect thyroid function.¹⁶ In a small study of healthy people, thyroid hormone (thyroxine) levels tended to be lower in those with lower blood levels of zinc. In people with low zinc, supplementing with zinc increased thyroxine levels.¹⁷ One case has been reported of a woman with severe zinc deficiency (caused by the combination of alcoholism and malabsorption) who developed hypothyroidism that was corrected by supplementing with zinc.¹⁸ Although the typical Western diet is marginally low in zinc,¹⁹ additional research is needed to determine whether zinc supplementation would be effective for preventing or correcting hypothyroidism.