Role of thyrotropin in metabolism of thyroid hormones in nonthyroidal tissues

T 4 conversion into T 3 in peripheral tissues is the major source of circulating T 3. However, the exact mechanism of this process is ill defined. Several in vitro studies have demonstrated that thyrotropin facilitates deiodination of T 4 into T 3 in liver and kidneys. However, there is a paucity of in vitro studies confirming this activity of thyrotropin. Therefore, this study was conducted to examine the influence of thyrotropin on thyroid hormone metabolism in nonthyroidal tissues. We assessed T 4, T 3, reverse T 3 (rT 3), and T 3 resin uptake (T 3RU) responses up to 12 hours at intervals of 4 hours in 6 thyroidectomized female mongrel dogs rendered euthyroid with LT 4 replacement therapy before and after subcutaneous (SC) administration of bovine thyrotropin (5 U) on one day and normal saline (0.5 mL) on another in a randomized sequence between 08:00 and 09:00 am. Euthyroid state after LT 4 replacement was confirmed before thyrotropin administration. Serum T 4, T 3, rT 3, and T 3RU all remained unaltered after SC administration of normal saline. No significant alteration was noted in serum T 3RU values on SC administration of thyrotropin. However, serum T 3 rose progressively reaching a peak at 12 hours with simultaneous declines being noted in both serum T 4 and rT 3 concentrations ( P < .05 vs prethyrotropin values for all determinations). The changes after SC administration were significantly different ( P < .001) in comparison to those noted on SC administration of normal saline. Thyrotropin may promote both the conversion of T 4 to T 3 and metabolism of rT 3 into T 2 in nonthyroidal tissues via enhancement of the same monodeionase.