Hormonal and environmental interactions on thyroid function in the chick embryo and posthatching chicken.

Area of Science:

• Endocrinology
• Developmental Biology
• Animal Physiology

Background:

• Thyroxine (T4) metabolism undergoes significant developmental changes.
• Peripheral monodeiodination of T4 is influenced by hormonal and environmental factors.
• Thyroid hormone profiles change near hatching, with increasing T3 and decreasing reverse T3 (rT3).

Purpose of the Study:

• To investigate developmental changes in thyroxine (T4) metabolism.
• To examine hormonal and environmental influences on peripheral T4 monodeiodination.
• To understand the regulation of thyroid hormones in response to environmental shifts.

Main Methods:

• Analysis of thyroid hormone concentrations (T4, T3, rT3) during development and under varying conditions.
• Administration of hormones (glucocorticoids, prolactin) prior to hatching.
• Exposure of posthatch chicks to different ambient temperatures (thermal acclimation).
• Investigation of feeding and fasting effects on thyroid hormone rhythms in hens.

Main Results:

• Hormonal administration (glucocorticoids, prolactin) induced developmental changes in T4 metabolism.
• Thermal acclimation altered T4 disappearance rate and peripheral conversion, with cold exposure favoring T3 production and heat favoring rT3.
• Peripheral T4 metabolism provides independent regulation alongside central hypothalamo-thyroid activity.
• Fasting in hens increased circulating T4 but decreased T3 by reducing 5'deiodinase activity and abolished the T3 circadian rhythm.

Conclusions:

• Peripheral T4 metabolism is a key regulatory pathway, adapting to environmental changes independently but in concert with central control.
• Thyroid hormone levels are fine-tuned by a balance of central and peripheral mechanisms, influenced by age, and environmental conditions.
• Feeding status significantly impacts thyroid hormone levels and rhythms, highlighting its role in overall regulation.