Free hormone assessment is a specialized diagnostic laboratory procedure that precisely measures the concentration of biologically active, unbound hormones circulating in the bloodstream. Unlike total hormone measurements, which quantify both protein-bound and free fractions, this assessment provides a more accurate reflection of the hormone available to engage with target tissue receptors and elicit a biological effect. Clinically, it is considered a superior metric for determining true hormonal status and the resulting tissue impact, especially for potent steroid and thyroid hormones.
Origin
The concept of measuring free versus bound hormones originated from the fundamental physiological understanding that many lipophilic hormones, including cortisol and testosterone, are transported primarily bound to carrier proteins like sex hormone-binding globulin (SHBG). The critical realization that only the unbound fraction is physiologically active led to the development of specific, high-sensitivity assays to isolate and quantify this “free” concentration, significantly improving diagnostic accuracy in endocrinology.
Mechanism
Hormones exist in two distinct circulatory pools: a large reservoir bound to carrier proteins, which are biologically inert, and a small, unbound, or “free,” fraction. Only the free hormone is capable of diffusing across capillary membranes and binding to its specific intracellular or cell-surface receptor to initiate a cellular response. The assessment mechanism typically employs techniques like equilibrium dialysis or ultrafiltration to physically separate the active free fraction from the protein-bound reservoir prior to analytical quantification.
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