IODINE AND THYROID-FUNCTION

Iodine is a raw material for the thyroid production of hormone which i s on the major external control of TSH. The thyroid adaptation to iodi ne deficiency consists in an increasing iodine concentration from the circulation, an enhancing iodination of the tyrosyl residues in thyrog lobulin, a decreasing iodine storage associated to a better recycling of non hormonal iodine and thyroid hyperplasia to provide more synthet ic possibilities. Genetic variation and environmental factors explain the wide variation of individuals response to iodine deficiency result ing in a high prevalence of goiter, a mild TSH level increase or overt hypothyroidism. At long term iodine deficiency may have severe pathol ogical consequences and induce neuropsychological deficits in school-c hildren. A policy of iodine supplementation mainly by iodized salt mus t be undertaken in many areas in the world. Effects of an iodine exces s on thyroid function are variable depending upon the underlying thyro id disorder and ambient iodine intake. The most subjects remain euthyr oid by mechanisms of autoregulation based on an inhibition of thyroid hormone synthesis and a decrease in the thyroide iodide trap. Euthyroi d individuals from high iodine intake areas or those with a history of lymphocytic thyroiditis, treated Graves' disease or subtotal thyroide ctomy develop hypothyroidism. On the other hand iodine induced hyperth yroidism is more common in areas of iodine deficiency and in older pat ients with nodular goiter.