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Thyroid hormone deiodination in birds.
Veerle M Darras1, Carla H J Verhoelst, Geert E Reyns
1Laboratory of Comparative Endocrinology, Zoological Institute, Leuven, Belgium. veerle.darras@bio.kuleuven.be
Thyroid : Official Journal of the American Thyroid Association
|February 21, 2006
Summary
Avian thyroid glands primarily secrete thyroxine (T4). Extrathyroidal tissues regulate active triiodothyronine (T3) availability through deiodination, involving enzymes D1, D2, and D3.
Area of Science:
- Endocrinology
- Comparative Physiology
- Molecular Biology
Background:
- Avian thyroid glands predominantly secrete thyroxine (T4).
- Active thyroid hormone 3,3',5-triiodothyronine (T3) availability in extrathyroidal tissues relies on deiodination.
- Birds possess three iodothyronine deiodinases (D1, D2, D3) analogous to mammals.
Purpose of the Study:
- To investigate the characteristics and regulation of iodothyronine deiodinases in avian species.
- To understand the tissue distribution, cellular localization, and developmental variations of these enzymes in chickens.
- To explore the influence of hormonal and environmental factors on avian deiodination processes.
Main Methods:
- Biochemical characterization of avian iodothyronine deiodinases.
- cDNA cloning of chicken deiodinase enzymes (D1, D2, D3).
- Analysis of enzyme activity and mRNA expression in various chicken tissues.
- Immunohistochemical studies for cellular protein localization.
- Investigation of regulatory effects from hormones and environmental factors.
Main Results:
- Avian deiodinases (D1, D2, D3) are biochemically and genetically similar to their mammalian counterparts.
- Detailed tissue distribution and cellular localization of deiodinases were established in chicken.
- Deiodinase abundance and distribution vary significantly during avian development.
- Hormonal axes (thyroid, growth hormone, glucocorticoids) and external factors (nutrition, temperature, light, pollutants) regulate deiodination.
Conclusions:
- Avian extrathyroidal deiodination is a critical mechanism for regulating T3 availability.
- Deiodinase enzymes (D1, D2, D3) play key roles in thyroid hormone metabolism across avian development.
- Deiodination is a complex process influenced by multiple endogenous and exogenous factors, finely tuning T3 levels.