Woodchucks (n = 12-14/group) with circannual cycles entrained to northern versus southern hemisphere photoperiods were assessed monthly for 16 mo. Changes in serum total triiodothyronine (TT₃), free thyroxine (T₄), total thyroxine (TT₄), and prolactin were determined in a subset of five animals per group. Metabolic hormone results were examined in relation to changes in body weight, food intake, and serum testosterone (n = 12-14/group). Seasonal changes in each parameter were similar in both groups as were nadir and peak TT₃ (162 ± 6 and 392 ± 12 ng/ml, respectively), free T₄ (19 ± 2 and 86 ± 7 ng/ml, repectively), TT₄ (3.2 ± 0.2 and 8.0 ± 0.5 ng/ml, respectively), and prolactin (0.6 ± 0.1 and 14 ± 2 ng/ml, respectively). In late winter and early spring, simultaneous increases in both free T₄ and prolactin were associated with 1) a large increase in food intake, 2) a decline in body weight to nadir values, 3) a corresponding negative energy balance, 4) a peak and decline in serum testosterone, and 5) a modest increase in TT₄ and large decline in serum TT₃. Low levels of free T₄ and prolactin were observed in summer when energy balance was very positive. The results demonstrate that, in woodchucks, serum T₄ and prolactin undergo seasonal changes during annual cycles entrained by photoperiod. The results suggest that changes in free T₄, acting as a calorigenic hormone, and changes in both T₄ and prolactin, potentially acting as lipolytic, antilipogenic, and/or orectic hormones, are likely involved in the mechanisms underlying the corresponding seasonal changes in food intake, fat metabolism, and energy balance in this species. Their potential roles in gonadal regression and recrudescence are less clear.