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  • Title: Free T4 and T3 in relation to total hormone, free hormone indices, and protein in plasma of rainbow trout and arctic charr.
    Author: Eales JG, Shostak S.
    Journal: Gen Comp Endocrinol; 1985 May; 58(2):291-302. PubMed ID: 3996892.
    Abstract:
    The percentages of total L-thyroxine (TT4) and total 3,5-3'-triiodo-L-thyronine (TT3) in the free form (%FT4 and %FT3) were measured in plasma from trout or charr by equilibrium dialysis at 12 degrees and by an index employing miniature G-25 Sephadex columns at room temperature. Dialysis estimates of both %FT4 and %FT3 were highly correlated with their respective column indices (rT4 = 0.84; rT3 = 0.92) validating the latter as a rapid and convenient method for monitoring alterations in plasma T4 or T3 binding. Binding depended on both pH and buffer composition. In charr, %FT4 (usual range 0.15-0.28%) exceeded %FT3 (usual range 0.09-0.17%); similar values were obtained for trout. The %FT4 values were much higher and the %FT3 values somewhat lower than those reported for mammals and birds. However, when TT4 and TT3 were taken into account, mean estimated concentrations of charr free T4 (FT4) and free T3 (FT3) were respectively 5.0 and 3.2 fmol/ml. These values are comparable to those in birds and mammals and suggest that FT4 and FT3 correspond more closely between various vertebrates than either TT4 and TT3 or %FT4 and %FT3. Plasma protein levels were correlated inversely with %FT4 (r = -0.54) and with %FT3 (r = -0.81) and correlated directly with TT4 (r = 0.45) and TT3 (r = 0.64). Despite these trends, FT4 and FT3 were highly correlated with their respective total hormone levels (r T4 = 0.95; rT3 = 0.94). Thus, for this charr population at least, TT4 and TT3 are representative of peripheral thyroidal status, as judged by the theoretically more physiologically relevant FT4 and FT3 levels.
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