The clinical term describing the fraction of an administered or naturally produced hormone that reaches the systemic circulation and is available to exert its biological effect on target tissues. This metric is crucial in endocrinology for determining the appropriate dosage and delivery route of hormone replacement therapy. High bioavailability ensures that a sufficient concentration of the active hormone can interact with its corresponding receptors.
Origin
“Bioavailability” is a pharmacological concept combining bio (life) and availability (capability of being used), originating in the study of drug absorption and distribution. When applied to hormones, it relates to the body’s natural processes of secretion, transport, and metabolic clearance, which dictate how much free, active hormone is present.
Mechanism
The mechanism is governed by the hormone’s route of administration, its solubility, and its susceptibility to first-pass metabolism in the liver or gut. For example, orally administered hormones often have lower bioavailability due to hepatic clearance, while transdermal or injectable routes bypass this initial metabolism. Transport proteins like Sex Hormone-Binding Globulin (SHBG) also influence the amount of unbound, biologically active hormone available to diffuse into target cells.
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