[Iodine and thyroid function]

Abstract

Iodine is a raw material for the thyroid production of hormone which is on the major external control of TSH. The thyroid adaptation to iodine deficiency consists in an increasing iodine concentration from the circulation, an enhancing iodination of the tyrosyl residues in thyroglobulin, a decreasing iodine storage associated to a better recycling of non hormonal iodine and thyroid hyperplasia to provide more synthetic possibilities. Genetic variation and environmental factors explain the wide variation of individuals response to iodine deficiency resulting in a high prevalence of goiter, a mild TSH level increase or overt hypothyroidism. At long term iodine deficiency may have severe pathological consequences and induce neuropsychological deficits in school-children. A policy of iodine supplementation mainly by iodized salt must be undertaken in many areas in the world. Effects of an iodine excess on thyroid function are variable depending upon the underlying thyroid disorder and ambient iodine intake. The most subjects remain euthyroid by mechanisms of autoregulation based on an inhibition of thyroid hormone synthesis and a decrease in the thyroid iodide trap. Euthyroid individuals from high iodine intake areas or those with a history of lymphocytic thyroiditis, treated Graves' disease or subtotal thyroidectomy develop hypothyroidism. On the other hand iodine induced hyperthyroidism is more common in areas of iodine deficiency and in older patients with nodular goiter.