What is the Origin of Free T/SHBG Ratio?

This ratio is derived from established principles of steroid and thyroid hormone transport dynamics within the serum compartment. "Free T" denotes the unbound, active thyroid hormone, while SHBG quantifies the primary transport protein. The calculation itself originates from laboratory methodologies designed to bypass the limitations of measuring only total hormone levels.

What is the Mechanism of Free T/SHBG Ratio?

The calculation functions by dividing the measured concentration of free thyroxine (FT4) by the measured concentration of SHBG, often normalized or indexed against reference ranges. A higher ratio suggests greater hormonal access to intracellular receptors, indicating increased potential for metabolic action. Conversely, a low ratio, even with normal total T4, may suggest functional hypothyroidism at the tissue level due to sequestration by binding proteins.

Free T/SHBG Ratio ∞ Area

Free T/SHBG Ratio

Meaning

The Free T/SHBG Ratio is a calculated index reflecting the biologically available fraction of circulating thyroid hormone relative to the amount bound by Sex Hormone-Binding Globulin (SHBG). This ratio offers a superior clinical metric compared to total T4 or T3 alone, as it more accurately represents the unbound hormone available to enter target cells and exert its metabolic effects. It is particularly informative when SHBG levels are pathologically high or low due to various physiological states.