The life durations of hypo- and hyperthyroid Wistar rats were measured under clean conventional conditions. The amount of exogenous T4 (thyroxine), which is sufficient to elevate T4 levels in the blood, decreased with age. The rats which were made hypothyroid by the neonatal T4 treatment had a longer lifespan than control. The lifespans of hyperthyroid rats, to which T4 solutions were given as drinking water during either the first or the second half of the life period, were shorter than control. The life-shortening effect of T4 was not detected when T4 was administered to already aged animals. These results indicate that the effect of T4 administration is not due to the direct promotion of the diseases which cause the death, but to the acceleration of aging during the young or middle-aged period.
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