What free testosterone is
Of all circulating testosterone, approximately 60% is tightly bound to SHBG (sex hormone binding globulin) and biologically inactive, ~38% is loosely bound to albumin (bioavailable), and only 1–3% circulates free — unbound to any protein. Free testosterone, along with albumin-bound testosterone, makes up "bioavailable testosterone."
Free testosterone can enter cells directly and activate the androgen receptor to drive gene expression — it is the functionally active hormone. SHBG-bound testosterone is effectively dormant and cannot enter cells.
Why free T matters more than total T for symptoms
Consider two men:
- Man A: Total testosterone 650 ng/dL, SHBG 65 nmol/L → calculated free testosterone ~8 pg/mL (low)
- Man B: Total testosterone 420 ng/dL, SHBG 22 nmol/L → calculated free testosterone ~14 pg/mL (normal)
Man A has a significantly higher total T but is functionally more testosterone-deficient. This scenario is common in older men, lean men with naturally high SHBG, and men on high-fibre diets. If you have symptoms of low testosterone but "normal" total T, always check free T and SHBG.
Measurement methods
Three methods exist for measuring free testosterone, with different accuracy levels:
- Equilibrium dialysis — gold standard; separates truly free testosterone; accurate but expensive and not widely available in India
- Direct immunoassay (free T kit) — most commonly offered by Indian labs; convenient but notoriously inaccurate, especially in women and older men; can over- or underestimate by 50%
- Calculated free testosterone (Vermeulen formula) — uses total testosterone + SHBG + albumin; reasonably accurate (within 10–15% of dialysis) and widely accessible; this is the best practical option for most people
If your lab offers only direct immunoassay free T, request total testosterone + SHBG instead and use an online Vermeulen calculator for a more accurate result.
What drives SHBG — up and down
| Factor | Effect on SHBG | Mechanism |
|---|---|---|
| Ageing | Increases significantly | Liver produces more SHBG; testosterone declines |
| Obesity / visceral fat | Decreases | High insulin suppresses SHBG production |
| Insulin resistance | Decreases | Elevated insulin downregulates hepatic SHBG synthesis |
| Hypothyroidism | Decreases | Low thyroid hormone reduces SHBG synthesis |
| Hyperthyroidism | Increases | Excess thyroid hormone upregulates SHBG production |
| Liver disease | Increases | Altered hepatic protein synthesis |
| High-fibre diet | Modest increase | Fibre reduces enterohepatic recycling of sex steroids |
| Anabolic steroids | Strongly decreases | Androgens suppress hepatic SHBG production |
How to lower SHBG naturally
Boron 10mg/day has the most direct evidence — a clinical study showed ~25–30% reduction in SHBG with 4 weeks of boron supplementation, alongside increased free testosterone and decreased estradiol. Boron inhibits the enzyme that converts free steroids to protein-bound forms.
- Boron: 10mg/day (glycinate or citrate form); well-tolerated; retest SHBG in 6–8 weeks
- Zinc: 15–30mg elemental zinc; may modestly lower SHBG in deficient individuals
- Vitamin D: Correcting Vitamin D deficiency improves SHBG through improved insulin sensitivity
- Resistance training: Improves insulin sensitivity → lower insulin → lower SHBG
- Managing insulin resistance: Berberine, myo-inositol, reduced refined carbohydrate intake
- Tongkat ali (Eurycoma longifolia): 200–400mg/day; may lower SHBG and increase free testosterone
Interpretation — free T and SHBG together
Always interpret free testosterone in the context of SHBG and total T. The goal is free T in the upper quartile of the reference range. If free T is low due to high SHBG, target SHBG reduction. If both free T and total T are low, the issue is primary (production).
The complete hormone panel
Free testosterone should never be tested in isolation. The complete picture requires: total testosterone, calculated free testosterone (via SHBG + albumin), SHBG, estradiol E2, LH, FSH, prolactin, and DHEA-S. This allows you to understand whether low T is testicular (primary), pituitary (secondary), or driven by elevated SHBG — each requires a different intervention.
Frequently asked questions
Why is my total testosterone normal but I feel low T symptoms?
High SHBG renders testosterone biologically inactive. Test free testosterone and SHBG alongside total T — high SHBG is the most common cause of symptomatic "low T" with normal total testosterone.
How do I lower SHBG naturally in India?
Boron 10mg/day has the strongest direct evidence (25–30% SHBG reduction in studies). Resistance training, reducing insulin resistance (berberine, dietary changes), zinc, and vitamin D correction all help. Retest SHBG at 6–8 weeks.
How do I calculate free testosterone from total T and SHBG?
Use the Vermeulen formula — requires total T (ng/dL), SHBG (nmol/L), and albumin (use 4.3 g/dL if not measured). Multiple free online calculators implement this. The result closely matches gold-standard equilibrium dialysis and is more reliable than direct immunoassay kits.
What is an optimal free testosterone level?
Free T should be in the upper quartile of the reference range. By direct assay, above 15 pg/mL is generally considered optimal for men under 50. Calculated free T above 15 ng/dL (Vermeulen) is a reasonable target. Context always matters — symptoms alongside numbers.