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Injections of triiodothyronine (T(3)) and thyroxine (T(4)) into chronically hypothyroid rats were used to evaluate the contribution of intracellular T(4) to T(3) conversion to nuclear T(3) in pituitary, liver, and kidney, and to correlate the occupancy of pituitary nuclear T(3) receptors with inhibition of thyroid-stimulating hormone (TSH) release. Injection of a combination of 70 ng T(3) and 400 ng T(4)/100 g body wt resulted in plasma T(3) concentrations of 45+/-7 ng/dl (mean+/-SD) and 3.0+/-0.4 mug/dl T(4) 3 h later. At that plasma T(3) level, the contribution of plasma T(3) to the nuclear receptor sites resulted in saturation of 34+/-7% for pituitary, 27+/-5% for liver, and 33+/-2% for kidney. In addition to the T(3) derived from plasma T(3), there was additional T(3) derived from intracellular monodeiodination of T(4) in all three tissues that resulted in total nuclear occupancy (as percent saturation) of 58+/-11% (pituitary), 36+/-8% (liver), and 41+/-11% (kidney), respectively. The percent contribution of T(3) derived from cellular T(4) added 41% of the total nuclear T(3) in the pituitary which was significantly higher than the contribution of this source in the liver (24%) or the kidney (19%). 3 h after intravenous injection of increasing doses of T(3), the plasma T(3) concentration correlated well with both the change in TSH and the nuclear occupancy, suggesting a linear relationship between the integrated nuclear occupancy by T(3) and TSH release rate. The contribution of intrapituitary T(4) to T(3) conversion to nuclear T(3) was accompanied by an appropriate decrease in TSH, supporting the biological relevance of nuclear T(3). Pretreatment of the animals with 6-n-propylthiouracil before T(4) injection decreased neither the nuclear T(3) derived from intrapituitary T(4) nor the subsequent decrease in TSH. These results indicate that intracellular monodeiodination of T(4) contributes substantially to the nuclear T(3) in the pituitary of the hypothyroid rat, and suggest a linear inverse relationship between nuclear receptor occupancy by T(3) in the pituitary and TSH release rate. The data further indicate that T(4) to T(3) monodeiodination is considerably more important as a source of nuclear T(3) in the pituitary than in the liver and kidney. This provides a mechanism whereby the TSH secretion could respond promptly to a decrease in thyroid secretion (predominantly T(4)) before a decrease in plasma T(3) would be expected to lead to significant metabolic hypothyroidism.
Silva et al. (Mon,) studied this question.