Effects of 3,5,3' -D-triiodothyronine on lipid metabolism in hypothyroidism

¹ Veterans Administration Hospital and Departments of Medicine and Biochemistry, Vanderbilt University, Nashville, Tennessee

To study the effects of 3,5,3'-d-triiodothyronine (d-T₃) on oxygen consumption and lipid metabolism, thyroid deficiency was produced by administration of I¹³¹ to weanling rats. Hypothyroidism was evidenced by decreased growth and O₂ consumption, with absence of functioning thyroid tissue at necropsy. Administration of d-T₃ produced a transient fall in serum cholesterol and a more protracted fall in serum fatty acids. This was associated with a decrease in liver fatty acids and an increase in liver cholesterol. Simultaneously, incorporation of radioactive carbon into liver fatty acids and cholesterol of d-T₃-treated animals was greatly increased. Total-body O₂ consumption and Qo₂ of myocardial homogenates were increased in animals treated with d-T₃. From these data it is suggested that the hypolipemic and calorigenic effects of d-T₃ are interdependent and administration at this dose level increases fatty acid utilization and cholesterol synthesis by the liver.