Effects of dietary caloric restriction and aging on thyroid hormones of rhesus monkeys

Plasma levels of thyroid hormones - triiodothyronine (T-3), thyroxin T-4), and thyroid-stimulating hormone (TSH) were measured in male and female rhesus monkeys (Macaca mulatta) fed either ad libitum or a 30% calorie-restricted (CR) diet (males for 11 years; females for 6 years). The same hormones were measured in another group of young male rhesus monkeys during adaptation to the 30% CR regimen. Both long- and shorter-term CR diet lowered total T-3 in plasma of the monkeys. The effect appeared to be greater in younger monkeys than in older counter-parts. No effects of CR diet were detected for either free or total T-4, although unlike T-3, levels of this hormone decreased with age. TSH levels also decreased with age, and were increased by longterm CR diet in older monkeys only. No consistent effects of shorter-term CR diet were observed for TSH. In the light of the effects of the thyroid axis on overall metabolism, these results suggest a possible mechanism by which CR diets may elicit their well-known beneficial 'anti-aging' effects in mammals.