What is the Origin of Thyroid Hormone Availability?

The term is fundamental to clinical endocrinology, stemming from the realization that total circulating hormone levels do not always reflect the true biological action due to transport protein binding and the necessity of peripheral conversion. The focus shifted from T4 production to the final, biologically relevant availability of free T3 at the cellular level.

What is the Mechanism of Thyroid Hormone Availability?

Availability is determined by a sequence of physiological steps. The thyroid gland produces T4, which is then transported via binding globulins. In peripheral tissues, deiodinase enzymes convert T4 into the active T3. Only the small fraction of T3 that is unbound is considered "available" to diffuse across the cell membrane and bind to the nuclear thyroid hormone receptor, which subsequently modulates gene transcription for metabolic regulation.

Thyroid Hormone Availability

Meaning

Thyroid Hormone Availability refers to the total quantity of active thyroid hormone, primarily triiodothyronine (T3), that is unbound and ready to interact with nuclear receptors in target tissues to regulate cellular metabolism. This concept encompasses the entire pathway, from the glandular production of thyroxine (T4) to its peripheral conversion into the active T3 and the final delivery of the free fraction to the cell. Adequate availability is essential for maintaining basal metabolic rate, body temperature, and optimal function of virtually every organ system.