Free plasma thyroxine levels in coho salmon, Oncorhynchus kisutch, during parr-smolt transformation: comparison with total thyroxine, total triiodothyronine, and growth hormone levels

Free plasma thyroxine (FT4) levels were measured in coho salmon, Oncorhynchus kisutch, during parr-smolt transformation (smoltification) using an equilibrium dialysis system followed by a radioimmunoassay. The FT4 data were correlated to total plasma thyroxine (TT4), triiodothyronine (TT3), and growth hormone (GH). Plasma samples were taken weekly from early April to late May, when the salmon where released from the hatchery. Free thyroxine and GH levels increased gradually through smoltification. TT4 levels increased significantly in mid-April and in mid-May. TT3 levels increased in April and remained elevated until late-May after which they declined to the lowest levels. During the first increase in TT4 levels, FT4 levels remained low and TT3 levels did not increase until FT4 levels increased in late-April. In addition, after TT4 levels decreased in late May, FT4 levels remained elevated. These data show that there are differences between the plasma FT4 and TT4 profiles during smoltification. Nevertheless, regression analysis indicates that FT4 levels are highly correlated to the increases in the levels of GH (r=0.73) and TT4 (r=0.70). In addition, GH is less correlated to TT4 and TT3 (r=0.24 and r=-0.46, respectively) compared with FT4 (r=0.73), suggesting a close relationship between the increases of FT4 and GH. In addition, these data suggest that this method of measuring free plasma thyroxine may provide a new tool for studying the timing of thyroid hormone action and regulation during parr-smolt transformation in salmonids.