Circulating thyroid hormone levels in recycled turkey breeder hens during a short day prelighting period and renewal of photosensitivity for egg production

Department of Poultry Science, College of Agriculture and Life Sciences, North Carolina State University, Raleigh, 27695-7608, USA. tom_siopes@ncsu.edu

Two experiments were conducted with yearling turkey hens at the end of their first season of egg laying. The purpose was to examine changes in plasma thyroid hormone levels during recycling and renewal of photosensitivity for egg production. Plasma concentrations of thyroid hormones were determined weekly or biweekly for 8 wk following a change from existing photoperiods of long days (LD) to short days (SD) and during the associated complete renewal (recycling) of photosensitivity for egg production. In both experiments, neither thyroxine (T4) nor triiodothyronine (T3) declined from starting values during the SD exposure but plasma T3 increased significantly from LD controls. There were no significant treatment effects (SD vs. LD) for T4 in either experiment. The increase in T3 was 101 and 145% in Experiments 1 and 2, respectively, and extended over the 8-wk SD treatment period in a parabolic fashion. The T3/T4 ratio was also significantly increased in the SD treatments of both experiments as compared to LD controls. Plasma concentrations of T3, but not T4, clearly changed during recycling and renewal of photosensitivity for egg production in breeder hens. These results were consistent with a participation of plasma T3 in promoting photosensitivity and diminishing photorefractoriness in turkey hens during SD-induced recycling.