Seasonal thyroid activity in the tammar wallaby, Macropus eugenii (Desmarest)

Abstract

Thyroxine secretion rate, the total and free thyroxine concentrations in plasma, and water turn- over rates were measured in six adult tammar wallabies throughout the year. There was a seasonal pattern of thyroid activity. The mean thyroxine secretion rate was low during the first summer, increased in autumn (April), and remained high through the winter months until October. In December it returned to the low summer level, approximately half that of winter. Water turnover was elevated during the hot summer months, indicating increased physiological demand of water for cooling. Mean plasma total thyroxine concentration followed a seasonal pattern similar to that of thyroxine secretion rate, high in winter, low in summer. Plasma free thyroxine gradually rose from a low initial summer concentration to reach a peak in October, then returned to the low summer level. These findings are consistent with a higher secretion of basal pituitary thyroid-stimulating hormone in winter than in summer.