SHBG (Sex Hormone Binding Globulin): What High or Low Levels Mean
The protein that controls how much testosterone and estrogen your body can actually use
Sex hormone binding globulin (SHBG) is a liver-produced protein that binds testosterone and estrogen, regulating how much of each hormone is bioavailable. High SHBG reduces free testosterone; low SHBG increases free estrogen — both cause symptoms.
SHBG is the master regulator of sex hormone bioavailability. Produced primarily by the liver, SHBG binds testosterone with high affinity (making it biologically inactive) and estrogen with somewhat lower affinity. Changes in SHBG levels directly alter the ratio of free-to-bound hormones — and therefore the symptoms you experience.
Understanding SHBG is essential because it explains many clinical scenarios where total hormone levels look normal but symptoms persist. A man with total testosterone of 600 ng/dL and SHBG of 70 nmol/L has less bioavailable testosterone than a man with total T of 450 ng/dL and SHBG of 25 nmol/L. Without measuring SHBG, you're only seeing half the picture.
SHBG Reference Ranges
| Population | Low | Optimal | High |
|---|---|---|---|
| Men (20–50) | <20 nmol/L | 20–50 nmol/L | >50 nmol/L |
| Men (50+) | <25 nmol/L | 25–60 nmol/L | >60 nmol/L |
| Premenopausal women | <30 nmol/L | 30–80 nmol/L | >80 nmol/L |
| Postmenopausal women | <25 nmol/L | 25–70 nmol/L | >70 nmol/L |
SHBG increases approximately 1.6% per year in men after age 30, steadily reducing free testosterone even when total T remains stable.
What Causes High SHBG
- Aging — SHBG rises progressively after age 30 in men
- Hyperthyroidism — thyroid hormones directly stimulate hepatic SHBG production
- Estrogen exposure — oral contraceptives, HRT, and environmental estrogens increase SHBG
- Liver disease — hepatitis and cirrhosis can increase SHBG production
- Low-calorie diets — caloric restriction raises SHBG, potentially reducing free T
- Anorexia nervosa — markedly elevated SHBG from malnutrition
- Anticonvulsants — carbamazepine, phenytoin increase SHBG via hepatic enzyme induction
What Causes Low SHBG
- Obesity / insulin resistance — insulin directly suppresses hepatic SHBG production. A HOMA-IR increase of 1 unit reduces SHBG by approximately 10%
- Hypothyroidism — low thyroid hormones reduce SHBG production
- Type 2 diabetes — hyperinsulinemia is a potent SHBG suppressant
- Polycystic ovary syndrome (PCOS) — low SHBG is both a feature and diagnostic marker
- Anabolic steroid use — exogenous androgens suppress SHBG
- Growth hormone excess — acromegaly lowers SHBG
Low SHBG in men means more free testosterone but also more free estradiol — which can cause gynecomastia, water retention, and mood changes. In women, low SHBG elevates free androgens, contributing to acne, hirsutism, and PCOS symptoms.
Frequently Asked Questions
What does high SHBG mean for men?
High SHBG in men reduces bioavailable (free) testosterone, potentially causing low-T symptoms — fatigue, reduced libido, loss of muscle mass — even when total testosterone is in the normal range. Common causes include aging, hyperthyroidism, liver conditions, and certain medications. If total T is normal but symptoms are present, check SHBG and free testosterone.
How can I lower SHBG?
If SHBG is elevated: address underlying causes (thyroid dysfunction, medications, liver issues). Nutritional strategies include adequate protein intake, zinc supplementation (30–45 mg/day), and maintaining sufficient caloric intake. Resistance training and maintaining healthy body composition help. For women on oral contraceptives, switching to non-oral forms may reduce SHBG.
Is SHBG included in standard bloodwork?
No. SHBG is not part of standard bloodwork panels. It must be specifically ordered (CPT code 84270). Most doctors test only total testosterone, which can appear normal even when free testosterone is low due to elevated SHBG. Always request SHBG alongside total testosterone for an accurate picture.
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