What is the Origin of Exogenous T3?

The term is a compound of the Greek prefix exo- meaning "outside" or "external" and the chemical abbreviation T3 for triiodothyronine. The pharmacological use of thyroid extracts and synthetic T3 has been a cornerstone of endocrinology since the early 20th century. Its clinical origin is tied to the need for effective, standardized replacement therapy for patients with thyroid gland insufficiency.

What is the Mechanism of Exogenous T3?

Upon ingestion, exogenous T3 is rapidly absorbed into the bloodstream and circulates, primarily unbound to carrier proteins. The T3 molecule then enters target cells and binds directly to the nuclear thyroid hormone receptor, a ligand-activated transcription factor. This binding event initiates the transcription of genes responsible for regulating basal metabolic rate, protein synthesis, and mitochondrial function, thereby exerting its potent metabolic and thermogenic effects throughout the body.

Exogenous T3 ∞ Area

Exogenous T3

Meaning

Exogenous T3 refers to Triiodothyronine, the active thyroid hormone, administered to the body from an external source, typically in the form of a pharmaceutical preparation like liothyronine. This is distinct from the body’s naturally produced, or endogenous, T3. Clinically, exogenous T3 is used in the treatment of hypothyroidism or in specific protocols where rapid metabolic effects or direct T3 replacement is required, bypassing the body’s natural conversion process from T4.