Regulation of rat thyroxine-binding globulin and transthyretin: studies in thyroidectomized and hypophysectomized rats given tri-iodothyronine or/and growth hormone.

We have investigated the role of the thyroid compared with the hypophysis in the regulation of the two saturable thyroid hormone carriers of rat serum, thyroxine-binding globulin (TBG) and transthyretin (TTR). We examined, at serum and hepatic mRNA level, the responses of TBG and TTR to thyroidectomy (Tx), hypophysectomy (Hx) and replacement treatments with tri-iodothyronine (T3) or/and GH, both hormones which are depleted when the thyroid or hypophysis are removed. The studies were performed on male rats at the age of 8 weeks, when the developmentally regulated TBG becomes undetectable after its transient postnatal rise, while the nondevelopmentally regulated TTR presents its normal, age-independent level of expression. Tx-induced TBG re-expression was completely reversed by T3 replacement and unresponsive to GH replacement. TTR in the serum, on the other hand, was not affected by Tx or T3 replacement, moderately reduced by Tx in terms of the amount of mRNA, and markedly reduced by GH replacement. GH treatment, moreover, inhibited the expression of TTR in euthyroid controls. Hx, like Tx, induced TBG re-expression, an effect efficiently antagonized by T3 replacement. However, TBG synthesis was higher in Hx than in Tx rats and less effectively antagonized by T3 replacement. Most unexpectedly, GH induced a dramatic further increase in TBG synthesis, and the TBG synthesized in the GH-replaced Hx rats was entirely resistant to down-regulation by T3 replacement. TTR was markedly decreased at both serum and hepatic levels by Hx, unaffected by T3 and further decreased by GH replacement.(ABSTRACT TRUNCATED AT 250 WORDS)