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 (FT 4 ) 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 FT 4 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.