Plasma total T3 and T4 concentrations in bucks as affected by photoperiod

In order to determine whether total thyroxine (T4) and 3-3 -5-triiodothyronine (T3) plasma concentrations are affected by photoperiodic changes in bucks, these hormones were assayed using RIA in samples collected weekly for a year period in Alpine and Saanen male goats exposed to natural photoperiod (46◦N latitude, control group), or to artificial photoperiod alternating every 2 months (4M group) or 1 month (2M) with long days (16L:8D) and short days (8L:16D). In the control group, the thyroid hormones showed marked seasonal variation. Maximal monthly peaks were recorded at the beginning of spring (April) for plasma T4 (35.7±1.8 ng/ml) and in late winter to early spring (March) for plasma T3 (0.91±0.11 ng/ml)—with the lowest concentrations being recorded in October (autumn) for plasma T4 (22.8±1.0 ng/ml) and in September (late summer to early autumn) for plasma T3 (0.42±0.01 ng/ml). In the 4M group, T4 plasma concentrations varied with daylength (p < 0.05), recording an overall mean concentration significantly (p < 0.01) higher during the short days (37.3±7.5 ng/ml), than during long days (32.2±7.3 ng/ml) with a significant effect of sampling time within each photoperiod (p < 0.0001). In both lighttreated groups, T3 varied with photoperiod (p < 0.0001) and time (p < 0.0001), increasing during long days and decreasing during short days. T3 plasma concentrations (derived from peripheral monodeiodination) followed the photoperiodic changes, while it was hypothesized that the stimulatory effect of daylength on T4 (thyroid gland secretory activity) was characterized by a longer latency period and could be seen in the 4M group during the following photoperiodic treatment. It is concluded that photoperiod affects thyroid hormone plasma concentrations in male goats, since different lighting regimes resulted in different profiles of both plasma T3 and T4. Bearing in mind the pivotal role of thyroid hormones in stimulating the metabolic activity of the whole body, it is possible that the light-induced increase in circulating hormones could sustain and improve animal production. The suitability of supplementary light treatments needs to be considered. © 2005 Elsevier B.V. All rights reserved.