Comparison of the Effects of Propylthiouracil and Sulfadiazine on Thyroidal Biosynthesis and the Manner in Which They Are Influenced by Supplemental Iodide1

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Abstract
Sulfadiazine (SD) and propylthiouracil (PTU) were administered to rats parenterally or in the diet for varying periods of time and over a wide dosage range. The effects of these drugs on thyroid weight and biosynthetic processes were compared. Both were qualitatively similar in their antithyroid activity. Coupling of iodinated tyrosines to form iodothyronines was most readily inhibited, then iodination of monoiodotyrosine, while iodination of tyrosine was much more resistant to inhibition. Supplementary iodide decreased the goiter produced by PTU but not that produced by SD. Iodide did not alter the effect of PTU on thyroidal biosynthesis but increased that of SD at least 1,000-fold. A marked potentiation of the inhibitory effect of SD was produced by as little as 10 ng KI daily, while a complete inhibition of all organic binding of iodine was produced with SD plus 100 jug KI daily. Iodide maximally potentiated the inhibitory effect of SD within 24 hours. No definite inhibition of thyroidal biosynthesis was produced by the administration of iodide alone. It is shown that iodide decreases thyroid size in PTU-treated rats primarily by increasing the amount of thyroid hormone formed.