Interrelationships between energy metabolism and thyroid hormone metabolism during starvation in the rat

Thyroid hormone metabolism and plasma concentrations of TSH were studied after short-term hypocaloric refeeding of rats starved for 2–6 days. Carbohydrate and protein (10 kcal) refeeding after 4 days of starvation resulted in a rapid increase in serum T3 (P<0.01) and, less consistently of T4. Plasma TSH did not change. These findings were not due to changes in the metabolic clearance rates or in thyroid hormone binding proteins, as the disappearance of injected labelled T3 and T4, and the free fractions of T3 and T4, were unchanged. Increased thyroidal secretion, and for T3, increased peripheral conversion from T4 were therefore responsible for these changes. Fat refeeding had no immediate effect on plasma T4, T3 or TSH. After 6 days of starvation, refeeding of any nutrient was ineffective in altering the plasma concentrations of T3 and T4. The intraperitoneal administration of nicotinamide (100 mg/100 g body weight) to starving animals caused an increase in blood glucose and a decrease in blood betahydroxybutyrate similar to that which followed carbohydrate refeeding; T3, however, did not increase. In spite of producing a profile of substrates in the serum similar to that found following carbohydrate refeeding, nicotinamide administration had no effect on the blood lactate/ pyruvate ratio which was increased following carbohydrate refeeding. Therefore, the cytoplasmic redox state, as reflected by the lactate/pyruvate ratio, may be closely related to the control of peripheral thyroid hormone metabolism.