SEX DIFFERENCE IN RESISTANCE OF RATS TO THYROID HORMONE

Abstract

Two series of experiments were performed to compare the metabolic responses of male and female rats to thyroid hormone. In the first series dried thyroid was fed in the diet at a concentration of 1.5 gm. per kgm. of chow. Initial body weights ranged from 130 to 160 gm. Oxygen consumption was measured with the rats lightly anaesthetized with pentobarbitone sodium. In the second series, sodium thyroxine was injected subcutaneously once a day for five days and oxygen consumption was measured without anaesthetic. Initial body weights averaged 147 gm. for males and 137 gm. for females. Two doses of sodium thyroxine were used, 0.1 and 0.5 mgm. per kgm. per day. In both series the increase in oxygen consumption following the administration of thyroid or thyroxine was much less for female rats than for the males. In Series I, half the animals were castrated. The castrated females responded to thyroid in the same way as the normal and castrated males. The castrated females were then given stilboestrol (5 mgm. per liter in drinking water) for three weeks while they were kept on the same diet containing thyroid. Their metabolic rate returned to the levels of unoperated females. Light anaesthesia during the measurement of oxygen consumption reduced the variance within groups with high rates of oxygen consumption but not in groups with normal rates.