Effect of Alterations in Thyroid Status on the Metabolism of Thyroxine and Triiodothyronine by Rat Pituitary Gland In Vitro

• Text
• PDF

Abstract

The metabolism of thyroxine (T4) was studied in slices of rat pituitary gland and liver from the same animal incubated in vitro with [125I]T4 and 10 mM dithiothreitol. In the pituitary gland, generation of 125I-labeled 3,5,3′-triiodothyronine (T3), as well as overall T4 degradation, increased significantly at 24 h after thyroidectomy and by 2 wk were approximately five times control values. Conversely, following a single injection of T3 (1.5 μg/100 g body wt), values for both functions were significantly decreased at 4 h, and reached a nadir of ∼20% of control values at 12 and 24 h. Net T3-neogenesis accounted for ∼70% of T4 degradation in control pituitaries from intact rats. This proportion was increased by thyroidectomy and decreased by T3 replacement. Indirect evidence indicated that thyroidectomy decreased, and T3 administration increased, non-T3 generating pathways of T4 metabolism, probably 5-monodeiodination leading to formation of 3,3′5′-triiodothyronine (rT3). As judged from studies by others, the prompt changes in T4 metabolism that followed thyroidectomy or T3 administration could not be explained by changes in pituitary cell type. Changes in T3-neogenesis in liver were the converse of those in pituitary, and were much slower to occur.

Authors

Michiko Maeda, Sidney H. Ingbar