Thyroid hormone and prolactin profiles in male and female turkeys following photostimulation.

The turkey hen, a photosensitive bird, will become photorefractory (PR) during the reproductive cycle and will cease laying despite a stimulatory day length. This response is thought to be "programmed" by hormonal events early in the reproductive cycle. The turkey tom, in contrast, produces semen for extended periods and has not been shown to exhibit PR. We compared hormone profiles following photostimulation of hens and toms to assess differences that might program one, but not the other, for PR. We photostimulated with 16 h light per day and measured plasma prolactin (PRL), thyroxine (T4), and triiodothyronine (T3) weekly for 12 wk, and again at 16 and 22 wk. Hens were fed ad libitum, and toms were moderately feed-restricted. Results showed increasing PRL levels following photostimulation in hens, with peak levels occurring at about the time of peak egg production, and declining thereafter. Toms maintained significantly lower concentrations of PRL (P < 0.0001) than hens after 2 wk of photostimulation. A highly significant sex by time interaction in plasma T3 levels was observed due to extreme fluctuations in males. Similar, often reciprocal, fluctuations in mean T4 concentrations also occurred in males. We recycled the toms and repeated blood collections under identical conditions, but with ad libitum feeding to determine if feed restriction may have produced these unusual results. This study revealed an initial significant decline in plasma T3 levels and an increase in T4 levels immediately following photostimulation, and then steady (T4) or slowly rising (T3) levels through 12 wk photostimulation. We conclude that PRL profiles of toms and hens differ markedly during the reproductive cycle, lending support to the suggestion that rising PRL may mediate the onset of PR. Further study is needed to determine if the low plasma T3 levels in males may be related to delayed PR. The extreme fluctuations in plasma T3 and T4 levels of toms receiving relatively mild feed restriction suggest a need for further study of the metabolic effects of feed restriction in turkeys.