What is the Origin of Free Hormone Index?

The concept originated in clinical chemistry and endocrinology, driven by the need for a more functional assessment of hormone activity. The term 'Free Hormone' explicitly contrasts with 'Total Hormone,' acknowledging the significant role of binding proteins in hormone transport and regulation. The index is a mathematically derived ratio, providing a practical estimate when direct measurement of the free fraction is complex or unavailable.

What is the Mechanism of Free Hormone Index?

Most steroid and thyroid hormones circulate bound to transport proteins; only a small percentage remains free. These carrier proteins act as a reservoir, regulating the immediate availability of the hormone to target cells. The Free Hormone Index is typically calculated by dividing the total hormone concentration by the concentration of the primary binding protein, offering a proxy for the concentration of the unbound, physiologically active hormone fraction that can engage target receptors.

Free Hormone Index ∞ Area

Free Hormone Index

Meaning

The Free Hormone Index is a calculated or measured clinical metric used to estimate the biologically active fraction of a specific hormone that is unbound to carrier proteins, such as Sex Hormone-Binding Globulin (SHBG) or albumin. Since only the free, unbound hormones can diffuse into tissues and bind to receptors to exert their physiological effects, this index is often a more accurate reflection of true hormonal bioavailability and tissue impact than total hormone levels. Clinicians use this index to assess functional hormone status, particularly for sex steroids and thyroid hormones.