Long-term food restriction depresses serum thyroid hormone concentrations in the rat.

Long-term food restriction exerts an anti-aging action in rodents. The mechanism underlying its modulation of aging processes is unknown but changes in endocrine systems have been postulated as couplers of food restriction to aging. The effects of long-term food restriction on the serum concentrations of thyroid hormones were examined in 6-month-old, male Fischer 344 rats. For 4.5 months a Food Restricted group was fed 60% of the amount consumed by an Ad Libitum group. Food restriction did not alter the 24-h mean T4 concentration but reduced the 24-h mean T3 concentration from 95 +/- 1 to 87 +/- 3 ng/dl. The Ad Libitum group exhibited diurnal rhythms in both serum T4 and T3 concentrations, with peak values for each hormone at 1000 h. Although food restriction eliminated the 1000 h peak for both thyroid hormones, it abolished the diurnal variation only for T3. The dietary-induced changes in serum T4 and T3 are consistent with a role for these hormones in the anti-aging action of food restriction.