Total testosterone includes two forms
Testosterone circulates mainly attached to proteins, including SHBG and albumin. A much smaller portion is unbound. A total testosterone test measures the combined amount of bound and unbound testosterone in a blood sample.
Free testosterone is the unbound portion
Free testosterone refers to testosterone that is not attached to a protein. It represents a small fraction of the total. Measuring or calculating it can be useful in selected situations, particularly when SHBG may be unusually high or low or when symptoms and total testosterone results do not agree.
Why SHBG matters
SHBG binds testosterone and helps regulate how much is available to tissues. Two people with the same total testosterone can have different free-testosterone estimates when their SHBG levels differ.
An SHBG result still does not diagnose a condition by itself. A clinician considers the assay, reference range, health conditions, medicines, and other laboratory findings.
Avoid interpreting isolated results
Online calculators and population averages cannot replace a clinical interpretation. The Endocrine Society recommends using accurate assays and appropriate reference ranges, and confirming consistently low morning testosterone before diagnosing hypogonadism.
If you receive a result that is outside or near the edge of a laboratory range, ask what it means for your specific symptoms and whether repeat or additional testing is appropriate.