To investigate the influence of exercise on thyroid metabolism, 46 healthy young regularly menstruating sedentary women were randomly assigned to a 3 x 2 experimental design of aerobic exercise and energy availability treatments. Energy availability was defined as dietary energy intake minus energy expenditure during exercise. After 4 days of treatments, low energy availability (8 vs. 30 kcal.kg body wt-1.day-1) had reduced 3,5,3'-triiodothyronine (T3) by 15% and free T3 (fT3) by 18% and had increased thyroxine (T4) by 7% and reverse T3 (rT3) by 24% (all P < 0.01), whereas free T4 (fT4) was unchanged (P = 0.08). Exercise quantity (0 vs. 1,300 kcal/day) and intensity (40 vs. 70% of aerobic capacity) did not affect any thyroid hormone (all P > 0.10). That is, low-T3 syndrome was induced by the energy cost of exercise and was prevented in exercising women by increasing dietary energy intake. Selective observation of low-T3 syndrome in amenorrheic and not in regularly menstruating athletes suggests that exercise may compromise the availability of energy for reproductive function in humans. If so, athletic amenorrhea might be prevented or reversed through dietary reform without reducing exercise quantity or intensity.