Influences of temperature and pH on free T4 and free T3 in charr and trout plasma.

The influences of temperature and pH on the percentages of T4 (L-thyroxine) or T3 (3,5,3'-triiodo-L-thyronine) in the free form (%FT4 or %FT3) were determined by equilibrium dialysis on Arctic charr and rainbow trout plasma. %FT4 and %FT3 of plasma from trout or charr acclimated at 12-13 degrees increased with dialysis temperature (5-19 degrees). When charr plasma was dialyzed at fish acclimation temperature (5, 13, 20 degrees), %FT4 and %FT3 also increased with temperature. However, when plasma from charr acclimated at 5, 13, or 20 degrees was dialyzed at 13 degrees, %FT4 and %FT3 showed no dependence on previous thermal acclimation. %FT4 of charr plasma increased with dialysis pH (7.0-7.8); %FT3 was less responsive to pH but was lower at pH 7.0-7.4 than at pH 7.6-8.0. It is concluded that acute in vitro physiologic changes in either temperature or pH alter the proportions of plasma T4 and T3 in the free form; comparable in vivo effects could explain aspects of environmentally modified T4 and T3 metabolism.