Effects of Dietary Caloric Restriction and Aging on Thyroid Hormones of Rhesus Monkeys

Plasma levels of thyroid hormones – triiodothyronine (T3), thyroxin (T4), 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 T3 in plasma of the monkeys. The effect appeared to be greater in younger monkeys than in older counterparts. No effects of CR diet were detected for either free or total T4, although unlike T3, 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.