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Effects of Season, Pregnancy and Lactation on Thyroxine Turnover in the Mare

University of California^,2, Davis 95616

Abstract

The effects of gestation, lactation and season on plasma thyroxine kinetics in the mare were studied following a single injection of ¹³¹I-thyroxine. The labeled thyroxine was recovered from the plasma by trichloroacetic acid (TCA), and LH-20 column purification methods. Plasma thyroxine concentrations were determined by a protein competitive binding technique. No significant difference was observed for the mean half-life values obtained by the TCA and LH-20 column methods. Thyroxine half-life was unaffected by season (winter, 47.2 hr. vs. summer, 54.4 hr.), pregnancy (non-pregnant, 54.4 hr. vs. early pregnancy, 58.6 hr. and nonpregnant, 47.5 hr. vs. late pregnancy, 45.5 hr.) or lactation (nonlactating, 54.4 hr. vs. lactating, 47.2 hr.). Acute cold exposure resulted in a 63% increase in plasma thyroxine concentration (37.8 ng/ml) compared with summer values (23.1 ng/ml). No significant difference of the plasma concentration means was observed between late winter (22.4 ng/ml) and summer (23.1 ng/ml) groups. No significant difference was observed in plasma thyroxine concentrations in early (15.3 ng/ml) or late (20.0 ng/ml) stages of gestation in pregnant mares compared with nonpregnant mare groups (23.1 and 22.4 ng/ml, respectively). Lactating mares had a significantly lower plasma concentration (16.5 ng/ml) compared with nonlactating mares (23.1 ng/ml). However, no significant difference of the plasma clearance rate means was observed for the early (3.0 µg/day per [body wt]) or late (3.3 µg/day per [body wt]) stages of gestation in pregnant and in lactating (4.2 g/day per [body wt]) mares compared with their respective control groups (4.4, 3.3 and 4.4 g/day per [body wt]).

¹ The authors wish to express their appreciation to P. G. Thompson, K. F. Senter and W. S. Tanaka for technical assistance.

² Department of Animal Science.