What is the Origin of Free Hormone Transport?

The term combines "free," meaning unbound, with "hormone transport," referencing the circulatory delivery system. This concept is central to endocrinology, distinguishing the functional hormone pool from the storage pool sequestered by globulins like SHBG or TBG. It addresses the active fraction of the endocrine signal.

What is the Mechanism of Free Hormone Transport?

Transport relies on the dynamic equilibrium between the hormone, its carrier protein, and the unbound state governed by the law of mass action. Since only a small percentage exists in the free state, small changes in carrier protein concentration or hormone binding affinity can significantly shift the free fraction. Tissues absorb the hormone directly from this free pool, initiating signal transduction within the cytoplasm or nucleus.

Free Hormone Transport ∞ Area

Free Hormone Transport

Meaning

Free Hormone Transport describes the physiological mechanism by which steroid hormones and thyroid hormones circulate in the bloodstream predominantly unbound to carrier proteins, making them immediately available to interact with target cell receptors. Only this unbound fraction is considered biologically active and capable of crossing the cell membrane to exert genomic or non-genomic effects. Clinical interpretation relies heavily on this distinction.