Iodine
Possible Deficiencies
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 1988–1994.16 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.
Side Effects
High amounts (several milligrams per day) of iodine can interfere with normal thyroid function and should not be taken without consulting a doctor.17 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.18 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.19 A possible link to thyroid cancer has been observed in areas where an iodine-rich diet is consumed,20,21 and among populations that supplement with iodine.22,23 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.24