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(3)RU) 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(3)RU all remained unaltered after SC administration of normal saline. No significant alteration was noted in serum T(3)RU 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.