Effects of glucocorticoids on plasma levels of thyroid hormones (T4 and T3) and testicular activity in catfish, Clarias gariepinus during different phases of annual breeding cycle.

Effects of short-term administration of corticosterone and cortisol on plasma levels of thyroid hormones, gonado-somatic index and testicular histology have been reported in catfish, Clarias gariepinus during different phases of its breeding cycle. Corticosterone administration had no significant effect on plasma levels of T4, T3 and T3/T4 ratio, irrespective of doses and phases of breeding cycle. However, 5 µg dose of cortisol significantly increased plasma levels of T3 and the T3/T4 ratio during quiescent and regressive phases, while it significantly decreased plasma levels of T4 during progressive phase. During breeding phase, 2 µg and 5 µg doses of cortisol significantly decreased plasma levels of T4 and T3, respectively, while 5 µg dose of cortisol alone reduced T3/T4 ratio. Irrespective of phases of annual breeding cycle and doses, short-term administration of corticosterone and cortisol had no significant effect either on GSI or testicular histology. These findings suggest that corticosterone is ineffective in stimulating plasma levels of thyroid hormones, while cortisol, depending on dose and phase/season, may differentially increase, decrease or have no effect on plasma levels of thyroid hormones in C. gariepinus.