Thyroid response to ovine thyrotropin challenge in cortisol- and dexamethasone-treated rainbow trout, Salmo gairdneri.

Cortisol or dexamethasone, administered to rainbow trout (Salmo gairdneri) in the form of intraperitoneal "implants" suspended in hydrogenated coconut oil, had no apparent effect on plasma total thyroid hormone levels, or plasma T3/T4 ratios. The duration of the ovine TSH-stimulated surge in plasma T4 levels was decreased and increased in fish given cortisol and dexamethasone "implants" respectively; plasma T3 levels were not affected by TSH stimulation. The level of steroids administered in the cortisol "implants" elicited a moderate increase in plasma cortisol levels, whereas plasma cortisol levels in the dexamethasone-treated fish were suppressed. The altered levels of plasma cortisol had no apparent effect on HSI, hepatic glycogen or hepatic lipid content.