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.