Residual effects of thyroid hormone on secretory activity of somatotroph population.

In the present experiments the effects of the interruption of a prolonged T3 treatment on somatotroph population and GH synthesis and secretion were studied. The treatment with pharmacological doses of T3 provokes marked ultrastructural changes in somatotrophs compatible with a stimulated synthesis of GH. These results can be correlated with the significant increase in pituitary GH content and the normal values of serum GH. Twenty-four hours after T3 withdrawal, somatotrophs exhibited a marked depletion of secretory granules by exocytosis. These changes were concurrent with a significant discharge of pituitary GH and a two-fold increase in GH serum levels. The serum concentrations attained the highest values on the second and the third day after the T3 suppression, while the pituitary GH contents recovered the control levels. Morphometry of somatotroph population revealed a clear proliferation of cells and increased areas immunostained for GH, 24 h after withdrawal of the T3 treatment. The effects of T3 on somatotrophs were persistent for several days and at least five days were required for all parameters to return to control values. These results provide a new insight on the residual activity of thyroid hormones on both functional activity and morphological organization of somatotrophs.