Variations in Plasma Thyroxine Concentrations Throughout One Year in Penned Sheep on a Uniform Feed Intake

Abstract

Plasma total thyroxine (T 4) concentrations were measured by radioimmunoassay throughout one year in 10 sheep maintained indoors under natural light and consuming a constant amount of feed. Plasma T4 concentrations varied throughout the year, the highest and lowest values occurring in the spring and autumn months respectively. The mean of all observations was 5 ·4±0·1 ,ug/dl. The changes observed did not appear to correlate with changes in ambient temperature, day length or with reported changes in wool growth.