Thyroxine concentrations in plasma of normal and hypophysectomized hagfish, Myxine glutinosa (Cyclostomata)

Abstract

The influence of the pituitary on thyroid function in hagfish was investigated using the direct approach of hypophysectomy. Plasma thyroxine (T₄) concentration, measured by competitive protein-binding analysis, was used as the index of thyroid activity. Two months postoperatively, T₄ levels were similar in hypophysectomized, partially hypophysectomized, and sham-operated fish, but the levels were reduced to about 50% of the titres in intact controls. No morphological changes were observed in the thyroid tissue of any of the operated groups of fish. The experiment provides no evidence for the existence of a pituitary thyrotropin in hagfish.