Thyroxine Secretion Rates and Plasma Cholesterol Levels of Young and Old Rats

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Abstract
Using the goiter-prevention assay, thyroxine secretion rates were determined in young (4.5 months) and old (29 months) female Wistar rats maintained at an environmental temperature of 26 [plus or minus] 1[degree]C. Young rats were estimated to secrete 2.4 [mu]g of l-thyroxine/100 g body weight daily, while the corresponding figure for old rats was 1.6 [mu]g. Expressed per 100 g body weight the daily 1-thyroxine secretion rate was estimated as 3.2 [mu]g for the young rats and 1.9 [mu]g for the old. It is tentatively suggested that the lower thyroxine secretion rate in the old rats may be due chiefly to a homeostatic adaptation to the greater effect which a given amount of thyroxine has on the tissues of the old female rat. Plasma cholesterol levels in the heart blood of the young normal rats were not significantly different from that of the old rats. Thiouracil significantly increased the plasma cholesterol levels in both young and old rats, but the effect was reliably greater in the latter. Physiological doses of thyroxine injected into thiouracil-fed young and old rats failed to inhibit the thiouracil-induced hypercholesterolemia even though the dosages used had simultaneously prevented the formation of goiter.