Caloric restriction but not exercise-induced reductions in fat mass decrease plasma triiodothyronine concentrations: A randomized controlled trial

Caloric restriction (CR) decreases circulating triiodothyronine (T-3) concentration. However, it is not known if this effect is due to body fat mass reductions or due to CR, per se. The purpose of this study was to test the hypothesis that plasma T-3 concentration decreases with CR-induced reductions in fat mass but not in response to similar decreases in fat mass that are induced by exercise. Sedentary, nonobese 50- to 60-year-old men and women with no clinical evidence of cardiovascular or metabolic disease and not taking thyroid medications were randomly assigned to 12 months of caloric restriction (n = 18) or exercise-induced weight loss (n = 17) or to a control group (n = 9). Body weight and composition and plasma concentrations of the thyroid hormones T-3, thyrotropin (TSH), thyroxine (T-4), and free thyroxine (FT4) were measured at baseline and 12 months. Fat mass changed significantly in the CR (-6.3 +/- 1.0 kg) and exercise (-5.5 +/- 1.0 kg) groups but not in the control group (-0.6 +/- 1.4 kg). The changes were not significantly different between the CR and exercise groups. Plasma T-3 concentration decreased in the CR group (-9.8 +/- 2.0 ng/dL, p < 0.0001) but not in the exercise (-3.8 +/- 2.1 ng/dL, p = 0.07) or control (-1.3 +/- 2.8 ng/dL, p 0.65) groups. TSH, T-4, and FT4 did not change in any of the study groups. Twelve months of CR decreased circulating T-3 concentrations in middle-aged adults. This effect does not appear to be attributable to changes in body fat mass because a comparable decrease in T-3 concentration was not observed in response to an exercise-induced fat mass reduction.