Thyroid hormone availability and activity in avian species: a review

Area of Science:

• Endocrinology
• Molecular Biology
• Animal Physiology

Background:

• Intracellular thyroid hormone (TH) availability is crucial for cellular function.
• TH metabolism involves deiodination and sulfation, influencing active hormone levels.
• Thyroid hormone receptors (TRs) mediate TH effects on gene transcription.

Purpose of the Study:

• To investigate the role of deiodination and enzyme balance in regulating intracellular TH availability.
• To examine the impact of food restriction and refeeding on TH metabolism in growing chickens.
• To understand the regulation of local T3 availability and its relation to TH activity.

Main Methods:

• Analysis of hepatic type III deiodinase (D3) levels in response to food restriction and refeeding.
• Measurement of plasma insulin-like growth factor-I (IGF-I) and T3 concentrations.
• Review of thyroid hormone receptor (TR) structure and function in gene regulation.

Main Results:

• Food restriction increased hepatic D3 levels and decreased plasma IGF-I and T3 concentrations.
• Refeeding rapidly restored hepatic D3 and plasma T3 to control levels.
• TRs bind to TH response elements (TREs) as dimers, modulating gene transcription.

Conclusions:

• Tissue- and time-dependent regulation of TH-activating and inactivating enzymes is essential for controlling local T3 availability.
• Enzyme balance plays a critical role in modulating TH activity and physiological responses.
• Unliganded TRs can also influence target gene transcription.