Sex Differences in in vivo Concentration of Labeled 1-Thyroxine by Rat Liver. ∗

Abstract

Summary

1. When labeled 1-thyroxine was given to rats, the concentration of “hormonal” (TCA-precipitable) radioactivity in liver was significantly greater in the female than in the male. 2. Castration did not alter these findings in either the male or female. The administration of estrogens to normal males and to castrated females, and the administration of androgens to normal females and castrated males were also without effect. However, hypophysectomy in females lowered values to those in normal or hypophysectomized males. 3. No significant difference in the radioactivity of kidney, blood, skeletal muscle and G.I. tract were found after 1-thyroxine administration, nor were urinary, fecal, or bile excretion rates of radioactivity significantly different. 4. Following administration of labeled 3,5,3' 1-triiodothyronine, no consistent differences in liver conncentration between male and female were found. 5. The mechanism responsible for the difference in the “thyroxine” concentration in the liver of male and female rats remains unknown.