[Peripheral conversion of T4 and T3 in man]

Area of Science:

• Endocrinology
• Thyroid Hormone Metabolism
• Human Physiology

Context:

• Thyroid hormones, including thyroxine (T4) and triiodothyronine (T3), play vital roles in regulating human metabolism.
• Understanding the peripheral metabolism and interconversion of these hormones is essential for comprehending thyroidal regulation and function.
• Previous research has focused on T4 as the primary thyroid hormone, with T3's role and production pathways requiring further elucidation.

Purpose:

• To investigate the peripheral metabolism of T4 and T3 in humans.
• To quantify the contribution of peripheral conversion of T4 to T3 production.
• To evaluate the hypothesis that T3 is the main effector of thyroid hormone physiological action.

Summary:

• Human studies demonstrate that triiodothyronine (T3) is secreted by the thyroid gland and also produced through peripheral conversion from thyroxine (T4).
• Calculations of disposal rates indicate that approximately 40% of daily metabolized T4 is converted into T3.
• Consequently, over 60% of the daily T3 production originates from this peripheral conversion of T4, supporting T3's primary role in thyroid hormone action.

Impact:

• Provides quantitative data on the peripheral conversion of T4 to T3 in humans.
• Reinforces the understanding of T3 as the principal biologically active thyroid hormone.
• Contributes to the knowledge base for endocrine research and clinical applications related to thyroid disorders.