A specialized diagnostic procedure that measures the concentration of hormones in the circulation that are unbound to carrier proteins, such as Sex Hormone-Binding Globulin (SHBG) or Albumin, and are thus physiologically active. Only the bioavailable fraction can readily diffuse into target tissues and bind to cellular receptors to exert a biological effect. Accurate assessment of this free hormone level provides a more clinically relevant picture of hormonal status than total hormone measurements alone. This testing is crucial for precise dosing in hormone optimization protocols.
Origin
The necessity for this testing arose from the fundamental understanding in endocrinology that most circulating hormones are protein-bound and therefore inert. The term combines the physiological concept of ‘bioavailability’ with standard ‘hormone testing’ to delineate a more functional measure. Clinical practice adopted this method to better correlate laboratory values with patient symptoms and therapeutic outcomes, particularly for sex steroids like testosterone and estradiol.
Mechanism
The measurement typically involves equilibrium dialysis, ultrafiltration, or calculation methods like the free androgen index (FAI) to separate the unbound fraction from the bound fraction. These techniques isolate the biologically active hormones, providing a direct metric of the signaling capacity available to the body’s cells. The results inform the clinician about the actual tissue exposure to the hormone, guiding adjustments to optimize the therapeutic index and minimize potential side effects.
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