MODIFICATIONS OF THYROID FUNCTION INDUCED BY CHRONIC ADMINISTRATION OF IODIDE IN THE PRESENCE OF »AUTONOMOUS« THYROID TISSUE

Abstract

Fourteen patients were studied before and during administration of a daily supplement of 0.5 mg of iodide over a period of 3 to 10 months. Only a slight increase in PBI was observed in three patients with a non-toxic goitre and in two myxoedematous patients under treatment. The same supplements of iodine induced a marked rise in PBI in four euthyroid patients with a typical thyroid »hot« nodule. In the four cases, the PBI values reached the thyrotoxic range after only a few weeks. These elevated values were maintained throughout the period of iodine administration; data for the thyroxine content of the serum were strictly correlated with the PBI values. The T₃-resin test measurements also gave values characteristic of hyperthyroidism. Later on, three of these patients developed definite symptoms of hyperthyroidism, although not until several months after the appearance of the biological picture. Finally a definite increase in PBI was also observed, albeit to a lesser extent, in five thyrotoxic patients previously treated with radioiodide or with methimazole. A progressive fall in the PBI was found in four patients after iodine supplements were discontinued. These observations indicate that chronic iodine supplementation leads to marked over-production of thyroid hormones in two pathological conditions characterized by the presence of autonomous thyroid tissue. It is concluded that, in the absence of an adequate feed-back control mechanism, the amount of dietary iodine may be one of the main regulatory factors involved in thyroid activity in human subjects.