Effect of fluorine on thyroidal iodine metabolism in hyperthyroidism.

Prolonged administration of a daily dose of 5-10 mg. of fluoride to patients with hyperthyroidism may cause clinical improvement together with a significant fall in the level of plasma protein-bound iodine and a reduction in the basal metabolic rate. Studies with radioactive fluorine failed to demonstrate any important accumulation of fluorine within the thyroid in vivo. Thyroidal, blood and urinary radioiodine studies suggest that fluorine inhibits the thyroid iodide-concentrating mechanism. Fluorine does not impair the capacity of the gland to synthesize thyroid hormone when there is an abundance of iodide in the blood. However, inhibition of the thyroidal concentrating capacity when the total iodide pool is low will impose a critical limitation of hormonal synthesis, and may explain the therapeutic effect. THE widespread use of fluoride to reduce dental caries has aroused considerable controversy because of the toxicity of this halogen. Under the conditions of relative iodine deficiency of Central Europe, a possible thyroid-inhibiting action of fluorine has elicited the interest of physicians and public health officers. Previous studies on animals and human beings did not demonstrate any significant and reproducible alterations of thyroidal function due to small doses of fluorine which did not exceed a daily intake of 2.0 mg (approximately the doses used for prophylaxis of dental caries). In man, however, administration of larger doses over a longer period of time caused a noticeable reduction of several parameters of thyroidal biologic activity (1-5). Several authors postulated that goitrous states may be attributed to fluorine intake and, conversely, that this element can be utilized in the treatment of hyperthyroidism (6-8). Other investigators, however, could not reproduce these definite changes in the thyroid, and thus the thyreostatic activity of fluorine is still questioned. Since the hyperfunctioning thyroid is a more sensitive structure than the normal gland, we studied the effect of fluorine on patients with hyperthyroidism. Few clinical studies Received February IS, 1958. * Present address: Assistant Professor of Physiology, Division of Basic Health Sciences, Emory University, Georgia, U.S.A.