The pituitary control of rat thyroxine binding globulin

Thyroxine-binding globulin (TBG) is in the rat a developmentally regulated protein, actively synthesized in postnatal developing pups and in aging animals, but undetectable in adults. Experimental depletion of thyroid hormones (TH) in adults by thyroidectomy (Tx) or by hypophysectomy (Hx) results in marked reexpression of TBG synthesis. T3 replacement in both cases antagonizes this effect, though only moderately in Hx rats. These observations point to a regulatory pathway of TBG synthesis common to TX and Hx rats, characterized by an inverse relationship between TBG and TH levels. However, along with this thyroid-dependent TBG regulation, important differences between Tx and Hx rats are evidenced, pointing to specific pituitary factors of TBG control. The most striking difference concerns the effect of growth hormone (GH) replacement: the TBG of Tx rats is not affected, in contrast to that of Hx rats which is further increased by GH administration. The TBG response of the GH-treated Hx rats, which is strongly resistant to inhibition by T3, might involve deshomeostasis of GH-controlled functions not necessarily linked to the thyroid, e.g. the pancreatic functions regulating carbohydrate or lipid metabolism. In the light of these studies, it may be envisaged that the high surge of rat TBG during postnatal development involves a transient sensitivity of the TBG gene to stimulation by endogenous GH, and that this sensitivity lasts until functional maturity of the hypothalamic-pituitary-thyroid axis has been achieved.