What is the Origin of Term 23?

The term is specific to thyroid endocrinology, discovered when researchers realized that T4 could be metabolized via two distinct deiodination pathways: one leading to the active T3 and another leading to the inactive rT3. The understanding of rT3's role emerged from the "euthyroid sick syndrome" concept, where non-thyroidal illness alters T4 metabolism to conserve energy. Its measurement is a crucial tool for assessing peripheral thyroid hormone utilization.

What is the Mechanism of Term 23?

The formation of rT3 is catalyzed by the Type 3 deiodinase enzyme (D3), which preferentially removes the iodine atom from the inner ring of the T4 molecule. This process is upregulated during periods of high cortisol, inflammation, or nutritional deficiency, effectively shunting T4 away from the active T3 pathway and toward the inactive rT3. High rT3 then competes with T3 for binding to the nuclear thyroid receptor, leading to a state of localized or systemic metabolic slowdown and reduced cellular energy production.

Term 23 ∞ Area

Term 23

Meaning

Reverse Triiodothyronine (rT3) is an inactive metabolite of the thyroid hormone thyroxine (T4), structurally similar to the active hormone T3 but possessing a mirror-image configuration that renders it biologically inert. It is formed by the deiodination of T4 in peripheral tissues, particularly during periods of physiological stress, caloric restriction, or systemic illness, as a protective mechanism to slow down metabolism. Clinically, elevated rT3 levels can occupy T3 receptors without activating them, effectively acting as an antagonist and contributing to symptoms of hypothyroidism despite normal TSH and total T4 levels.