What is SHBG?
💡 SHBG binds sex hormones, making them inactive. Low SHBG = more free (active) testosterone = amplified androgen effects. Normal female: 40–120 nmol/L. Low SHBG (<40 nmol/L): associated with insulin resistance, PCOS, obesity. FAI = (testosterone × 100) / SHBG — the best measure of androgen excess. High SHBG: elevated oestrogen, OCP use, hyperthyroidism.
SHBG (sex hormone-binding globulin) is a liver-produced protein that binds testosterone and oestradiol in the blood, rendering them biologically inactive. Only unbound (free) hormone exerts biological effects. In fertility medicine, SHBG is measured to calculate free androgen index (FAI) — the true measure of bioavailable testosterone — and is often reduced in insulin-resistant, obese, and PCOS patients, amplifying androgen effects.
🇮🇳 India Context: SHBG is widely assessed and treated across major Indian fertility centres including Chennai, Mumbai, Bangalore, Delhi, and Hyderabad.
What are the key characteristics of SHBG?
- Produced by the liver; tightly binds testosterone (with high affinity) and oestradiol (moderate affinity); albumin binds hormones with low affinity
- Normal female range: 40–120 nmol/L (varies with OCP use — OCP dramatically raises SHBG)
- Low SHBG (<40 nmol/L): more free testosterone → amplified androgenic effects despite "normal" total testosterone; associated with insulin resistance, obesity, type 2 diabetes, hypothyroidism
- High SHBG (>120 nmol/L): less free testosterone; seen with oestrogen therapy, OCP use, hyperthyroidism, liver disease, eating disorders (low BMI)
- Free Androgen Index (FAI) = (total testosterone ÷ SHBG) × 100: FAI >5 in women = biochemical androgen excess (regardless of total testosterone level)
- PCOS biochemical signature: elevated total/free testosterone + low SHBG + elevated FAI — this combination is the most sensitive and specific biochemical marker of PCOS hyperandrogenism
- Insulin resistance → low SHBG: elevated insulin directly suppresses hepatic SHBG production → more free testosterone → worsens PCOS hyperandrogenism in a positive feedback loop
- SHBG in men: measured when testosterone is borderline; low SHBG + normal total testosterone = adequate free testosterone (no hypogonadism); high SHBG + normal total = may have low free testosterone
How does SHBG work?
Why does SHBG matter in fertility?
SHBG is an essential component of the PCOS androgen workup — total testosterone alone frequently misses androgen excess because low SHBG amplifies biological activity without raising total testosterone. The FAI (calculated from testosterone + SHBG) is the most clinically sensitive measure of androgen excess. In obese PCOS women, SHBG can be severely suppressed, making the FAI markedly elevated even when total testosterone is borderline. The most actionable implication: low SHBG + high FAI in a PCOS patient signals significant insulin resistance — the primary intervention should be weight loss and metformin (which raises SHBG over 3–6 months), potentially restoring ovulation without needing IVF. A woman who raises her SHBG from 20 to 60 nmol/L through weight loss and metformin has effectively "treated" her androgen excess at the source.
What are related terms to SHBG?
Testosterone
Testosterone is an androgen hormone measured in fertility evaluations for both s…
DHEAS (Dehydroepiandrosterone Sulfate)
DHEAS (dehydroepiandrosterone sulfate) is an androgen produced primarily by the …
PCOS (Polycystic Ovary Syndrome)
PCOS is a common hormonal disorder where the ovaries produce too many male hormo…
Estrogen (Estradiol)
Estrogen (specifically Estradiol or E2) is the primary female reproductive hormo…
Thyroid Function Tests
Thyroid function tests are a panel of blood tests assessing thyroid health — pri…
FAQs about SHBG
What is SHBG in a fertility blood test?
SHBG (sex hormone-binding globulin) is a protein produced by the liver that binds testosterone and oestradiol, making them biologically inactive. Only unbound (free) hormone acts on tissues. In fertility testing, SHBG is used to calculate the Free Androgen Index (FAI) — the most accurate measure of biologically active testosterone. Low SHBG amplifies androgen effects; high SHBG reduces them.
What does low SHBG mean for fertility?
Low SHBG (<40 nmol/L) means more free, biologically active testosterone — amplifying androgen effects even if total testosterone is only borderline elevated. Common causes: insulin resistance, obesity, PCOS, hypothyroidism. In fertility terms: low SHBG + elevated FAI = PCOS biochemical hyperandrogenism. Primary treatment: insulin sensitisation through weight loss and metformin, which raises SHBG over 3–6 months and normalises androgen excess — potentially restoring ovulation without IVF.
How is SHBG used to calculate free testosterone?
Free Androgen Index (FAI) = (total testosterone nmol/L × 100) ÷ SHBG nmol/L. Example 1: testosterone 2.0 nmol/L, SHBG 20 → FAI = 10.0 (elevated — androgen excess). Example 2: testosterone 2.0 nmol/L, SHBG 80 → FAI = 2.5 (normal). FAI >5 in women = biochemical androgen excess. This is why total testosterone alone is insufficient — always calculate FAI when assessing hyperandrogenism in PCOS.
Does the OCP affect SHBG?
Yes — significantly. Combined oral contraceptive pills (especially those containing ethinylestradiol) raise SHBG 2–4-fold. This dramatically reduces free testosterone — which is why OCP effectively reduces hirsutism and acne in hyperandrogenic women. After stopping the OCP, SHBG can remain elevated for 3–6 months, causing paradoxically low free testosterone. OCP-elevated SHBG can confuse post-pill androgen assessments — wait at least 3 months off OCP before a definitive baseline SHBG measurement.
Can raising SHBG improve fertility in PCOS?
Yes — directly. In insulin-resistant PCOS women, elevated insulin suppresses hepatic SHBG production, creating a vicious cycle: low SHBG → high free testosterone → worsened anovulation. Raising SHBG through insulin sensitisation (weight loss, metformin, dietary changes) reduces free androgen index and can restore spontaneous ovulation in many PCOS women. A 5–10% weight loss in obese PCOS restores ovulation in ~50% without medication. This makes SHBG measurement one of the most actionable parameters in PCOS fertility management.
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