Summary
Sex Hormone Binding Globulin (SHBG) is a protein made by the liver that binds tightly to testosterone and oestradiol, controlling how much is free and biologically active. Because SHBG determines the bioavailable fraction of these hormones, it is essential for interpreting testosterone and oestrogen results — and is used to calculate free testosterone and the free androgen index.
Only hormone that is not bound to SHBG can act on tissues. High SHBG therefore reduces the active fraction of testosterone and oestradiol, while low SHBG increases it. This means two people with identical total testosterone can have very different active hormone levels depending on their SHBG.
SHBG is influenced by many factors: it rises with ageing, hyperthyroidism, liver disease, and oestrogen; it falls with obesity, insulin resistance, type 2 diabetes, hypothyroidism, and excess androgens. Low SHBG is itself a marker of insulin resistance and metabolic risk.
SHBG is measured alongside total testosterone (and albumin) to calculate free testosterone and the free androgen index — giving a far more accurate picture of androgen status than total testosterone alone.
What It Is
SHBG is a homodimeric glycoprotein synthesised primarily by hepatocytes. Each molecule binds one steroid with high affinity, preferentially binding dihydrotestosterone and testosterone more avidly than oestradiol. SHBG-bound steroid is biologically inactive; only free and loosely albumin-bound steroid is available to tissues.
Hepatic SHBG production is regulated by hormonal and metabolic signals: oestrogen and thyroid hormone increase it; insulin and androgens decrease it. Low SHBG is strongly associated with insulin resistance, visceral adiposity, and increased risk of type 2 diabetes — independent of other markers.
Reference ranges: men 18.3–54.1 nmol/L; women 17.3–125 nmol/L (higher in women, and rising further with oestrogen use). SHBG is required to calculate free testosterone and the free androgen index (FAI = total testosterone ÷ SHBG × 100).
Functions
Hormone transport and buffering
Binds and transports testosterone and oestradiol in the blood, buffering their availability to tissues.
Bioavailable hormone regulator
Determines the free, active fraction of sex hormones — high SHBG lowers active hormone, low SHBG raises it.
Free testosterone calculation
Required, with total testosterone and albumin, to calculate free testosterone and the free androgen index.
Metabolic risk marker
Low SHBG is an independent marker of insulin resistance and increased risk of type 2 diabetes.
Reference Ranges
Sex Hormone Binding Globulin (SHBG)
Measured in nmol/L| Status | Range (nmol/L) | What it means |
|---|---|---|
| Low | < 18 (men) / < 17 (women) | Low SHBG — raises free hormone; associated with insulin resistance, obesity, and hypothyroidism. |
| Normal | 18–54 (men) / 17–125 (women) | Normal binding capacity — appropriate hormone bioavailability. |
| High | > 54 (men) / > 125 (women) | High SHBG — lowers free hormone; seen with ageing, hyperthyroidism, liver disease, and oestrogen. |
Reference ranges are sex-specific and influenced by age, thyroid status, liver function, and oestrogen use. SHBG is most informative when interpreted with total testosterone to assess free hormone.
Symptoms of Imbalance
SHBG itself causes no symptoms; its effects are seen through altered availability of testosterone and oestrogen.
- Symptoms of androgen excess in women (acne, hirsutism) due to raised free testosterone
- Features of insulin resistance and metabolic syndrome
- Often associated with central weight gain
- Menstrual irregularity in women with PCOS
- Symptoms of low free testosterone in men (low libido, fatigue) despite normal total testosterone
- Reduced bioavailable hormone effects
- May reflect hyperthyroidism or liver disease
Causes of Imbalance
- Obesity and insulin resistance
- Type 2 diabetes
- Hypothyroidism
- Excess androgens (including anabolic steroids)
- PCOS
- Cushing's syndrome
- Ageing
- Hyperthyroidism
- Liver disease (cirrhosis, hepatitis)
- Oestrogen (oral contraceptives, HRT, pregnancy)
- Anorexia and very low body weight
- Certain anticonvulsant medications
FAQs
SHBG controls how much of your testosterone is free and active. A normal total testosterone can hide a low free testosterone if SHBG is high, explaining symptoms that the total level alone would miss. Conversely, low SHBG raises the active fraction. Measuring SHBG alongside total testosterone (and albumin) allows free testosterone to be calculated, giving a much more accurate picture.
Low SHBG is most commonly caused by obesity, insulin resistance, and type 2 diabetes — insulin directly suppresses SHBG production in the liver. Hypothyroidism, excess androgens (including anabolic steroids), and Cushing’s syndrome also lower it. Because of its link with insulin resistance, a low SHBG is itself a useful marker of metabolic risk.
SHBG rises with ageing, hyperthyroidism, liver disease, and oestrogen exposure (oral contraceptives, HRT, and pregnancy). High SHBG binds more testosterone, lowering the free, active fraction — which can cause symptoms of testosterone deficiency in men even when their total testosterone appears normal.
Oestrogen, particularly when taken orally, raises SHBG significantly. In women, this can reduce free testosterone and occasionally contribute to lowered libido. It can also affect interpretation of androgen levels. Understanding your SHBG helps put hormone results in context, especially when starting or changing oestrogen-containing treatments.
References
- Hammond GL. Plasma steroid-binding proteins: primary gatekeepers of steroid hormone action. J Endocrinol. 2016;230(1):R13–R25. View source
- Ding EL, et al. Sex hormone-binding globulin and risk of type 2 diabetes. N Engl J Med. 2009;361(12):1152–1163. View source
- Vermeulen A, et al. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666–3672. View source