Total testosterone, free testosterone, and SHBG—without the jargon

Most testosterone travels with a carrier

Testosterone circulates in several forms. Much of it is bound tightly to sex hormone-binding globulin, or SHBG. Another portion is bound more loosely to albumin, and a small fraction circulates unbound as free testosterone.

A total testosterone test counts the hormone across those forms. It is the standard starting measurement because it is widely available and has the most established clinical interpretation.

Why SHBG can change the picture

Two people with the same total testosterone can have different amounts of calculated or measured free testosterone if their SHBG concentrations differ. Age, thyroid status, liver conditions, body composition, medications, and other factors may influence SHBG.

Clinical guidelines therefore recommend considering free testosterone in selected situations—particularly when total testosterone is close to a decision threshold or when a condition that changes SHBG is present. The calculation and assay method matter, so the numbers should be interpreted by a qualified clinician.

• Total testosterone: the usual first measurement
• SHBG: a major binding protein that changes availability
• Free testosterone: the small unbound fraction

What this means for pattern tracking

A wellness tracker should be explicit about which biological fraction its assay is intended to reflect. A convenient measurement is not automatically interchangeable with a laboratory total or free testosterone result.

Axis is exploring repeated saliva-based pattern tracking. Establishing what that signal represents—and how reliably it relates to validated laboratory methods—is part of the R&D work still required.