Effects of thyroidectomy on egg production, molt, and plasma thyroid hormone concentrations of turkey hens.

Turkey hens were thyroidectomized at different ages and phases of their reproductive cycles. Egg production, primary remige molting, and plasma thyroxine (T4) and triiodothyronine (T3) concentrations were monitored during each of three experiments. Thyroidectomy of hens (26 and 30 wk of age) prior to photoinduction of egg production prevented subsequent egg production and molting. Thyroidectomy of adult hens (36 wk of age) early in the egg production cycle caused a gradual termination of egg laying without the occurrence of molting. Thyroidectomy of mature hens (66 wk of age) during the light restriction period prior to their second egg production cycle prevented subsequent egg laying and molting. All successful thyroidectomies reduced plasma T4 and T3 to concentrations below the sensitivity levels of the assays. Feeding 1 ppm T4 to thyroidectomized hens resulted in resumption of normal egg production and molting. These results indicate that the thyroid is essential for the initiation and maintenance of egg production and for molting in turkey hens.