Seasonal influences on the control of plasma sex hormone-binding globulin by T4 in male little brown bats.

Male little brown bats showed a well-defined seasonal cycle of plasma thyroxine (T4). Total T4 concentrations increased during hibernation, reached peak levels in May at spring emergence, and subsequently declined to lowest levels in September. Thyroxine-binding globulin (TBG) activity increased only in summer, and seasonal changes in non-TBG-bound T4 generally paralleled those of total T4. During the fall and early hibernation period, bats studied under feral conditions or following photoperiod manipulation showed coincident changes in plasma T4 and sex hormone-binding globulin (SHBG) levels. In contrast, males studied during late hibernation or following spring arousal exhibited declining plasma T4 levels during periods of increased SHBG activity. Additionally, the prehibernation decline in SHBG did not depend on changing environmental conditions. Bats maintained under summerlike conditions during the fall showed normal seasonal reductions in SHBG and T4. These studies indicate that T4 is the likely physiological regulator of plasma SHBG activity during the fall and early hibernation, but that other factors modulate the timing of SHBG induction in the spring.