Nutritionally-induced alterations in thyroid hormone metabolism and thermogenesis.

Serum 3,5,3'-triiodothyronine (T3) is increased by overnutrition and decreased by starvation. The production rate of T3 was increased by overfeeding (82%), but T4 production did not change. Feeding a weight maintenance diet with high CHO mimiced serum T3 and rT3 changes of overfeeding, substitution of CHO by FAT those of starvation. VO2 increased during overfeeding and in T4 equilibrated volunteers supplemented with 40 microgram T3 daily for 2 weeks. No weight change occurred in the latter subjects. Yet no decrease in VO2 was found when serum T3 was reduced by the administration of iopanoic acid. VO2 decreased during starvation in normal and hypothyroid rats, and in hypothyroid animals replaced with T4 or T3.