Iodine

Since the introduction of iodized salt, iodine supplements are unnecessary and not recommended for most people. For strict vegetarians who avoid salt and sea vegetables, 150 mcg per day is commonly supplemented. This amount is adequate to prevent a deficiency and higher amounts are not necessary.

Seafood, iodized salt, and sea vegetables—for example, kelp—are high in iodine. Processed food may contain added iodized salt. Iodine is frequently found in dairy products. Vegetables grown in iodine-rich soil also contain this mineral.

People who avoid dairy, seafood, processed food, and iodized salt can become deficient. Iodine deficiency can cause low thyroid function, goiter, and cretinism. Although iodine deficiencies are now uncommon in Western societies, the U.S. population has shown a trend of significantly decreasing iodine intake from 1988–1994.¹ If this trend continues, iodine deficiency diseases may become more common.

Severe iodine deficiency during critical periods of brain development can lead to physical abnormalities and profound mental impairment. Little is known about the effects of mild iodine deficiency on neurological development and cognitive function. Iodine deficiency has become more common in New Zealand because if the lower concentration of iodine in milk resulting from the discontinuation of the use of iodine-containing sanitizers in the dairy industry and because of the declining use of iodized salt along with increasing consumption of processed foods made with non-iodized salt. 184 children (aged 10 to 13 years) in Dunedin, New Zealand, were randomly assigned to receive, in double-blind fashion, 150 mcg/day of iodine or placebo for 28 weeks. At baseline, children were mildly iodine-deficient (median urinary iodine concentration, 63 mcg/L; thyroglobulin concentration, 16.4 mcg/L). After 28 weeks, iodine status improved in the supplemented group (urinary iodine concentration, 145 mcg/L; thyroglobulin, 8.5 mcg/L), whereas the placebo group remained iodine-deficient. Iodine supplementation significantly improved scores on 2 of 4 tests of cognitive function assessed. The overall cognitive score was significantly greater in the iodine group than in the placebo group (p = 0.011). Iodine supplementation had no significant effect on the serum total T4 level. It is concluded that iodine supplementation improved cognitive function in mildly iodine-deficient children, and that mild iodine deficiency could prevent children from attaining their full intellectual potential.

Common iodine supplement preparations include potassium iodide, potassium iodate, and a combination of iodine and potassium iodide, otherwise known as Lugol’s solution.²

Children with iodine deficiency may also have iron-deficiency anaemia, and this anaemia may interfere with the therapeutic action of iodine supplementation.³ Correcting iron deficiency in such children with iron supplements has been shown to improve the efficacy of oral iodine in treating goitre.⁴

High amounts (several milligrams per day) of iodine can interfere with normal thyroid function and should not be taken without consulting a doctor.⁵ Although potassium iodide supplementation (prescribed for some skin disorders) is usually well-tolerated, it has been known to produce adverse reactions such as rashes, itching or lesions on the skin, gastro-intestinal symptoms, or hypothyroidism, especially in people with a prior history of thyroid problems.⁶ Because of such potential problems, the use of potassium iodide therapy should be supervised by a doctor. The average diet provides about four times the recommended amount of iodine. For susceptible people, that amount of iodine may be enough to cause health problems.⁷ A possible link to thyroid cancer has been observed in areas where an iodine-rich diet is consumed,^{8, 9} and among populations that supplement with iodine.^{10, 11} However, there is insufficient evidence to conclude that iodine supplementation is responsible for the increased incidence of thyroid cancer. Some people react to supplemental iodine, the first symptom of which is usually an acne-like rash.

When people with small, nontoxic goitre (living in areas not deficient in iodine) received iodine injections, they had a higher incidence of abnormal antibodies suggestive of the early stages of autoimmune thyroid disease.¹²