Turkey plasma thyroid hormone and prolactin concentrations throughout an egg laying cycle and in relation to...

Plasma concentrations of thyroxine (T4), triiodothyronine (T3), and prolactin were determined weekly through an egg laying cycle induced with 30 wk of 14 h light:10 h dark. Egg production, primary remige molting, and occurrence of photorefractoriness were monitored. Hens were assigned to one of two groups; those that exhibited photorefractoriness (PR) and those that remained photosensitive (PS). Weekly plasma T4 concentrations relative to the time of initiation of photostimulation did not differ between PR and PS groups; concentrations peaked prior to the onset of egg production, were lowest during peak egg production, and increased between Weeks 6 and 30 of photostimulation. Plasma T4 levels of PR hens peaked 3 wk prior to the onset of photorefractoriness. Plasma T3 concentrations of PS and PR hens were similar until increases occurred in PR hens at the onset of photorefractoriness. Plasma prolactin levels in PS and PR hens increased after photostimulation, declined sooner in PR hens, and were generally greater in PS than PR hens during the last 20 wk of the study. Mean time of onset of photorefractoriness in PR hens was 23.0 wk (range = 18 to 28 wk) after initiation of photostimulation. Egg production of PS hens was greater than that of PR hens after 20 wk of photostimulation, and PR hens molted a greater number of primary remiges than PS hens. Results indicated that 1) elevated plasma T4 may be involved in development of photorefractoriness in turkey hens, 2) increases in plasma T3 may be involved in gonadal regression and molting, and 3) elevated plasma prolactin was associated with persistence of photosensitivity.