Effects of L- and D-Thyroxine on Cholesterol Synthesis and Turnover in the Chick. ∗

Summary

The effect of L- and D-thyrox-ine on cholesterol synthesis and turnover in thyroidectomized and in normal untreated chicks has been compared using acetate-2-C¹⁴ and cholesterol-4-C¹⁴. (a). Although liver weights and liver cholesterol levels were not significantly different between the normal (EU) and iodoamino acid treated thyroidectomized chicks and those thyroidectomized without treatment, serum cholesterol levels were elevated only in the last group, (b). Appearance of labeled serum cholesterol after acetate-2-C¹⁴ injection, cholesterol synthesis after incubation of liver slices with acetate-2-C¹⁴, and cholesterol-4-C¹⁴ turnover in these various groups indicate that both L-thyroxine and D-thyroxine in the quantities used can stimulate cholesterol biosynthesis and turnover in a manner comparable to that found in the normal, (c). The data are discussed in the light of present concepts of the mode of action of thyroid substances on cholesterol metabolism, and the applicability of the findings to mammalian species.