Sex Hormone-Binding Globulin

SHBG is a protein that binds testosterone and oestradiol in the blood. Hormone bound to SHBG is not biologically available — only the free and weakly-bound fractions act on tissues. SHBG therefore determines how much of a given total testosterone is actually doing anything.

This is the single most common reason testosterone results are misinterpreted. SHBG is not a constant: it falls with obesity, insulin resistance, and hypothyroidism, and rises with age, oestrogen, and hyperthyroidism. A man with obesity can have a LOW total testosterone and a NORMAL free testosterone — because his SHBG is low, not because he is androgen-deficient. Measuring total testosterone alone in exactly that man, declaring him deficient, and selling him therapy is a well-established commercial pattern.

1. 1

   FoundationHormonal HealthHow to read hormones properly — and why both over-treatment and under-treatment cause harm.
2. 2

   ConditionMale HypogonadismClinically low testosterone — confirmed, not assumed
3. 3

   Signal RecordTestosterone TherapyTestosterone replacement for male hypogonadism (TRT)
4. 4

   Body SystemHormonesEndocrine signaling that regulates the whole body.
5. 5

   BiomarkerTestosteroneTotal and free — the primary androgen

1. Measure it
2. Understand your result
3. See what raises and lowers it
4. Apply evidence-based interventions
5. Recheck on your review cadence

An orientation to how this topic is typically approached — not medical advice.

SHBG is what converts a total testosterone result into a meaningful one. When SHBG is low — common in obesity and insulin resistance — total testosterone underestimates true androgen status, and free testosterone should be measured or calculated. When SHBG is high — common with age — total testosterone can look reassuring while free testosterone is genuinely low. Either way, a total testosterone interpreted without SHBG is a number without a context.

• Serum SHBG
• Always alongside total testosterone
• Free testosterone (measured or calculated) where SHBG is abnormal

SHBG's role in determining bioavailable sex hormone is established physiology, and guidelines specifically recommend measuring it — or measuring free testosterone — when total testosterone is borderline or when conditions that alter SHBG (notably obesity) are present.

• Endocrine Society Clinical Practice Guideline: Testosterone Therapy in Men with Hypogonadism

Sources are named authoritative guidelines and scientific bodies. Biomarker pages present established clinical context; calibrated evidence verdicts on specific interventions live in the linked Signal Records.