Gain of function TSH receptor mutations and iodine deficiency: implications in iodine prophylaxis

Iodine deficiency is widely known to be the main cause of nodular goiter (NG). In iodine deficient areas subclinical and overt hyperthyroidism is the major cause of morbidity and it is mainly due to toxic NG rather than Graves' disease. Toxic NG, including toxic multinodular goiter and toxic thyroid adenoma is usually encountered in subjects with long-standing NG, in whom thyrotoxicosis is usually preceded by a long phase of euthyroidism and then subclinical hyperthyroidsm (abnormally low TSH with normal circulating thyroid hormones). Epidemiological studies indicate that, compared to Graves' disease, the incidence and prevalence of non-autoimmune hyperthyroidism due to toxic adenoma and toxic multinodular goiter differ in different regions of the world, being much more frequent in areas of iodine deficiency. Recently, mutations of the TSH receptor (TSHr) gene causing permanent activation of the thyroid follicular cell adenylate-cyclase, have been shown to be the most probable cause of the hyperfunction and growth of toxic adenoma. In this review we will focus our attention on the role of external factors (i.e. iodine deficiency) with respect to individual factors (i.e. genetic mutations) in the pathogenesis of toxic NG.