Circannual changes in free thyroxine, prolactin, testes, and relative foodintake in woodchucks, Marmota monax

Woodchucks (n = 12-14/group) with circannual cycles entrained to northern v ersus southern hemisphere photoperiods were assessed monthly for 16 mo. Cha nges in serum total triiodothyronine (TT3), free thyroxine (T-4), total thy roxine (TT4), and prolactin were determined in a subset of five animals per group. Metabolic hormone results were examined in relation to changes in b ody weight, food intake, and serum testosterone (n = 12-14/group). Seasonal changes in each parameter were similar in both groups as were nadir and pe ak TT3 (162 +/- 6 and 392 +/- 12 ng/ml, respectively), free T-4 (19 +/- 2 a nd 86 +/- 7 ng/ml, respectively), TT4 (3.2 +/- 0.2 and 8.0 +/- 0.5 ng/ml, r espectively), and prolactin (0.6 +/- 0.1 and 14 +/- 2 ng/ml, respectively). In late winter and early spring, simultaneous increases in both free T-4 a nd 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 incre ase in TT4 and large decline in serum TT3. Low levels of free T-4 and prola ctin were observed in summer when energy balance was very positive. The res ults demonstrate that, in woodchucks, serum T-4 and prolactin undergo seaso nal changes during annual cycles entrained by photoperiod. The results sugg est that changes in free T-4, acting as a calorigenic hormone, and changes in both T-4 and prolactin, potentially acting as lipolytic, antilipogenic, and/or orectic hormones, are likely involved in the mechanisms underlying t he corresponding seasonal changes in food intake, fat metabolism, and energ y balance in this species. Their potential roles in gonadal regression and recrudescence are less clear.