RESPONSE OF THE THYROID GLAND TO THIOCYANATE AND THYROTROPIN*†

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Abstract
In order to determine whether inhibition of the thyroidal iodide "trap" by thiocyanate was dependent only upon the serum concentration of the ion, thyroid function was studied in 15 euthyroid and 3 hyperthyroid subjects before and during administration of potassium thiocyanate. Serum thiocyanate levels were correlated with thyroidal radioiodine (ll31) uptakes, and thyrotropin was administered to 8 euthyroid and 3 hyperthyroid subjects while the thyroid gland was under the inhibitory influence of thiocyanate. In euthyroid subjects complete interference with thyroidal I13l accumulation did not occur readily at serum thiocyanate levels below 5 mg per 100 ml. In hyperthyroid subjects, thyroidal I131 uptake proved impossible to inhibit completely, regardless of the magnitude of the serum thiocyanate level. In euthyroid subjects, at all serum thiocyanate concentrations, thyrotropin (20 to 300 U.S.P. units) produced measurable increments in the collection of radioiodine by the thyroid. In contrast, the hyperthyroid subjects exhibited much less response to thyrotropin. Aside from the distinct differences between euthyroid and hyperthyroid subjects in their response to thiocyanate and thyrotropin, the data suggest that the mechanism by which thiocyanate inhibits the thyroidal iodide "trap" may not be solely a function of extracellular concentration, since the thyroidal incorporation of radioiodine in response to thyrotropin may occur in the presence of an otherwise inhibitory serum level of thiocyanate.