Decreased T4 to T3 conversion in tissues of streptozotocin-diabetic rats

The effect of experimental diabetes on T4 to T3 conversion, T3-deiodination, and the pituitary response to a dose of T4 and T3 was studied. Pituitary GH and plasma TSH were determined as a measure of the biological response to thyroid hormones. Thyroidectomized rats, 5 days after injection with saline or streptozotocin (thyroidectomized-control (Th.C) and thyroidectomized-diabetic (Th.D) rats, respectively) received an ip dose of T4 + [125I]T4 or T3 + [125I]T3. Rats from each group were sacrificed at varying intervals after thyroid hormone injection. Th.D rats had hyperglycaemia, glycosuria, and a body weight of about 80% of that of Th.C rats. The concentrations of [125I]T4 and [125I]T3 were measured in several tissues after ethanol extraction, separation by thin-layer chromatography, and identification with markers. Plasma TSH and pituitary GH were measured by specific RIAs. Diabetes decreased the conversion of T4 to T3 in several tissues, including the pituitary, but did not affect the deiodination of T3. The decrease in pituitary T3 content after a dose of T4 was accompanied by a diminution of the biological effect of the T4 dose on pituitary GH. Since diabetes also interferes with this biological response to a T3 dose, it seems likely that the reduced biological effect of thyroid hormones on pituitary GH may be related to an alteration in the somatotrophin T3 receptors, or in post-receptor events. Moreover, the data indicate that although T3 generation in the pituitary was reduced, the same dose of T4 had a greater inhibitory effect on TSH secretion in Th.D rats than in Th.C rats.(ABSTRACT TRUNCATED AT 250 WORDS)