What is the Origin of Free Testosterone Availability?

The concept originates from clinical endocrinology and steroid biochemistry, following the recognition that a significant portion of circulating steroid hormones is protein-bound and biologically inert. The measurement of "free availability" became necessary to accurately diagnose and manage conditions like hypogonadism, providing a clearer picture of tissue-level hormone action. The term emphasizes the functional aspect of the hormone.

What is the Mechanism of Free Testosterone Availability?

Testosterone is synthesized primarily in the testes and ovaries, then released into the circulation where it binds extensively to plasma proteins, particularly SHBG. The small, unbound fraction of free testosterone passively enters target cells, where it can bind to intracellular androgen receptors or be converted into dihydrotestosterone (DHT) or estradiol (E2). Factors like insulin levels, liver function, and thyroid status directly influence SHBG production, thereby modulating the free testosterone availability and its subsequent biological signaling.

Free Testosterone Availability

Meaning

Free Testosterone Availability refers to the concentration of biologically active, unbound testosterone circulating in the bloodstream, representing the fraction of the hormone immediately available to diffuse into target tissues and interact with androgen receptors. Unlike total testosterone, this free fraction is not bound to Sex Hormone Binding Globulin (SHBG) or albumin, making it the primary driver of androgenic effects on muscle, bone, libido, and mood. It is the most clinically relevant measure of functional androgen status.