[Modulation of T4 to T3 conversion. Comparative aspects (author's transl)]

Area of Science:

• Comparative endocrinology
• Thyroid hormone metabolism
• Vertebrate physiology

Background:

• Peripheral conversion of thyroxine (T4) to triiodothyronine (T3) is a conserved process across vertebrate evolution.
• The presence and role of reverse triiodothyronine (rT3) remain less understood in certain vertebrate groups, including fish and amphibians.
• Thyroid hormone metabolism is crucial for development and adaptation, as seen in amphibian metamorphosis and fish adaptation to different salinities.

Purpose of the Study:

• To investigate the role of peripheral deiodination in thyroid hormone regulation across vertebrates.
• To explore the presence and significance of rT3 in various vertebrate species.
• To understand the specific mechanisms regulating T4 to T3 conversion in fish, particularly the influence of prolactin.

Main Methods:

• Comparative analysis of thyroid hormone metabolism in primitive and evolved vertebrates.
• Investigation of T4 and T3 synthesis and conversion pathways.
• Assessment of hormonal influences, such as prolactin, on deiodination processes.

Main Results:

• Peripheral T4 to T3 conversion is evident in primitive vertebrates like larval lampreys and more evolved species.
• rT3 has not been detected in fish, and its role in amphibians requires further clarification, though it appears significant in chick embryos.
• T4 to T3 conversion is upregulated during amphibian metamorphosis and in saltwater fish, coinciding with increased thyroidal secretion.
• In fish, prolactin directly stimulates T4 5-deiodination, independent of the pituitary-thyroid axis.

Conclusions:

• Peripheral deiodination of thyroid hormones is a fundamental process in vertebrates, with variations in rT3 presence and function.
• Prolactin plays a direct role in regulating T4 deiodination in fish, highlighting a unique endocrine pathway.
• Understanding these pathways is crucial for comprehending vertebrate adaptation, development, and physiological regulation.