Factors involved in the control of plasma thyroid hormone concentrations during an induced molt in the domestic hen

Egg production in the domestic hen improves following a molt. Experiments were directed at developing a method of inducing molt and comparing it with the traditional method of temporary feed withdrawal (FW). Molting mechanisms were also studied. Molt was induced by infusion of a gonadotropin releasing hormone agonist (GnRH-A; (D-Leu$\sp6$,Pro$\sp9$) -GnRH N-ethylamide), and compared with molt induced by FW. Hens were divided into three groups: (1) sham infused (controls); (2) molted by GnRH-A infusion (GnRH-A); or (3) molted by FW. Plasma was assayed for T$\sb4$, T$\sb3$, and P$\sb4$, and egg production was recorded.Plasma T$\sb4$ concentrations were elevated (p $<$.01), while plasma T$\sb3$ did not change and plasma P$\sb4$ decreased (p $<$.05) in the GnRH-A group compared to the control group. Plasma T$\sb4$ was higher (p $<$.05) in the GnRH-A group compared to the FW group, while plasma T$\sb3$ was elevated (p $<$.05) in the FW group compared to the GnRH-A group. Plasma P$\sb4$ concentrations declined to similar low levels in the GnRH-A and FW groups. The GnRH-A molted hens lost less (p $<$.001) body weight (BW) and were out of lay for 12 days less than FW molted hens.Endocrine changes associated with GnRH-A induced ovarian regression in the laying hen were examined. Measurement of elevated plasma concentrations of T$\sb4$ and T$\sb3$ were repeated in 2 experiments with different bleeding regimens. GnRH-A did not affect ovarian P$\sb4$ production in vitro, suggesting lack of a direct ovarian action of GnRH-A. There were no differences in plasma GH and LH concentrations in birds with ovaries regressed by GnRH-A compared to controls, although LH concentrations did appear to decline.Changes in the plasma thyroid hormone binding capacity of plasma during GnRH-A treatment were examined using either charcoal precipitation, gel filtration, or equilibrium dialysis. All 3 techniques only detected binding of low affinity and high capacity. It is concluded that only thyroid hormone binding of low affinity is present in hen plasma. No detectable changes in the low affinity binding of T$\sb4$ by hen plasma occurred during induced ovarian regression.U of I OnlyETDs are only available to UIUC Users without author permissio