“…The concept of “euthyroid hypometabolism,” based on the notion that triiodothyronine is “the peripherally active form of thyroxine,” has yet to be established (60, 61), although it is interesting to speculate about in connection with the reported decrease in the rate of disappearance of radiothyroxine in senescence. A study in rats showed no prolongation of life when thyroxine was administered and a slight increase in mortality during the actual period of administration (62). In another study thyroid was administered to old rats for ten months, after which they were studied at autopsy (63, 64).…”