Relative biopotency of triiodothyronine and of thyroxine for inducing oxygen consumption in young chicks

An experiment was conducted to evaluate the relative biopotency of triiodothyronine (T3) and of thyroxine (T4) for inducing oxygen consumption in young chicks. Male chicks, Single Comb White Leghorn, at 40 days of age were distributed into five experimental groups, one control group of 6 birds and four hormone-treated groups of 3 chicks. The chicks were injected intraperitoneally with T3 or T4 at a dose of 500 or 1,000 micrograms per kg of body weight. Thereafter, oxygen consumption was measured by indirect calorimetry using a respiration test apparatus. Oxygen consumption was significantly increased at 2 and 4 h for the birds treated with T3 or T4 compared with the control birds, except for the chicks treated with T4 at 500 micrograms at 2 h. From the coefficients for a multiple regression equation of oxygen consumption on T3 and T4, the data showed that T3 was 2 to 3 times as biologically active as T4.