Variations in plasma thyroxine concentrations throughout one year in penned sheep on a uniform feed intake

Plasma total thyroxine (T4) 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 microgram/dl. The changes observed did not appear to correlate with changes in ambient temperature, day length or with reported changes in wool growth.