Environmental salinity and the thyroidal response to thyrotropin in juvenile coho salmon (Oncorhynchus kisutch).

The effect of environmental salinity on the thyroidal response, measured by plasma concentrations of thyroxine (T4), to exogenous bovine thyrotropin (bTSH) was assessed early and late in the smoltification of coho salmon (Oncorhynchus kisutch). A single injection of 100 mIU bTSH significantly increased plasma T4 concentrations in fish in both fresh water and seawater, and both early (March 19) and late (May 24) in smoltification. In fish transferred to seawater, the increase in T4 titers was somewhat earlier and its duration shorter, suggesting an environmentally induced change in the kinetics of T4 entry and exit from the blood stream. The overall response to TSH was greater near the beginning (March 19) of the smoltification-associated T4 increase than near the end (May 24), suggesting a development-related change in thyroid function.