THE EFFECT OF PROPYLTHIOURACIL ON THE CONVERSION OF MONOIODOTYROSINE TO DIIODOTYROSINE1

The ratio of monoiodotyrosine to diiodot3Tosine (MIT⁄DIT) and triiodothyronine to thyroxine (T3⁄T4) was increased by feeding rats propylthiouracil before the injection of I131. This change occurred even when the uptake of I131 by he thyroid had returned to normal by allowing 24 hours or more to elapse after stopping propylthiouracil or by using small amounts of propylthiouracil. A depletion of total iodine in the gland was not the cause since the MIT⁄DIT remained elevated when total iodine was preserved by adding thyroid to propylthiouracil. A high thyrotropin did not account for the result since the added thyroid would have suppressed this. A high intrathyroidal iodide was not the explanation sincethe addition of KSCN made no difference. Thyroid alone increased MIT⁄DIT probably by inhibiting the thyrotropin essential to the conversion of monoiodotyrosine to diiodotyrosine. The results indicated that the conversion of monoiodotyrosine to diiodotyrosine was more sensitive to propylthiouracil than the iodination of tyrosine to form monoiodotyrosine. A block of the conversion of monoiodotyrosine to diiodotyrosine by propylthiouracil with a resultant increase in triiodothyronine relative to thyroxine, would help to account for the observed high radioiodine uptakes in hyperthyroid patients made euthyroid by propylthiouracil.