Plasma thyroxine levels in freshwater perch: influence of season, gonadotropins, and gonadal hormones

Abstract

Plasma thyroxine (T₄) of a freshwater female perch Anabas testudineus (Bloch) has been measured by specific radioimmunoassay (RIA). Monitoring of plasma T₄ throughout the year showed that July to early September (spawning stage) had the highest T₄ and late September to January (postspawning stage) had the lowest T₄. The plasma T₄ cycle was coincident with the ovarian cycle. Treatment of intact perch with salmon gonadotropin (SG-G100) or ovine LH (500 ng/100 g body wt) markedly enhanced plasma T₄ levels. Ovine FSH was without any effect. Ovarian steroids, estrone and 17β-estradiol (100 ng/100 g body wt), also significantly augmented T₄ level whereas estriol or progesterone had no such effect. There was a striking fall of plasma T₄ in ovariectomized fish. The drop of T₄ due to ovariectomy could not be reversed by SG-G100 but administration of estrone or 17β-estradiol to ovariectomized fish effectively reversed the drop of plasma T₄ level. These findings suggest that gonadotropin's stimulatory effect on perch thyroid is mediated via the release of ovarian steroids.