This study examined whether free (f) triidothyronine (T3), f thyroxine (T4), thyroid stimulating hormone (TSH), and leptin concentrations at rest changed in response to 20 weeks of exercise-training. Two groups of women were recruited for participation in the study, collegiate athletes ( n=17) and sedentary controls (n=4). Exercise training consisted of daily athletic activity such as rowing, running, and weight lifting. Subjects were initially grouped into rowers and controls. However, earlier suggested criteria were further used to categorize hormone changes (percentages) in the subjects into (+) responders (increases), (-) responders (decreases), or non-responders (no changes). The fT3 results of the rowers revealed two distinct categories of responses, (-) responder (all decreases; n=10) and non-responder (no change; n=7) rowers. In the responders fT3 concentration decreased (P<0.05) from baseline (BL) during an intense training period [(mean SEM) at 5 weeks by -28.2 (6.2)% and at 10 weeks by -24.9 (7.9)%], then returned towards BL levels (20 weeks compared to BL, P>0.05). Similar changes (P<0.05), at comparable times, were noted for leptin and TSH concentrations in the (-) responder rowers. The non-responder rowers and control subjects displayed no significant (P>0.05) hormone changes over the 20 weeks. The hormone changes observed in the (-) responder rowers were not significantly (P>0.05) correlated with changes in body composition or hydration status during the study. The mechanism for the hormone changes in the (-) responder rowers is unclear. We speculate the decrease in concentrations of TSH and fT3 could be attributable to a lower hypothalamic-pituitary signaling action, and this is related to the decreased leptin concentrations, and could represent a possible means of energy conservation in these exercising women.