Sex hormone-binding globulin is a carrier protein released by the liver. It binds testosterone with high affinity, holding it firmly, whereas albumin binds testosterone only loosely.1 The testes make a closely related protein, androgen-binding protein, but that stays inside the testes rather than entering the blood.1
Because SHBG holds on so tightly, it acts like a reservoir. It keeps a stock of hormone in circulation while limiting how much is immediately available as free testosterone. It also binds DHT and oestradiol.
For most men, SHBG only becomes interesting when a testosterone result looks odd. A high or low SHBG can make total testosterone look better or worse than the amount the body can actually use.
What makes SHBG high or low?
Age, body weight, metabolic health, thyroid function, liver health and several medicines all shift SHBG. Obesity and insulin resistance push it down, while ageing and an overactive thyroid push it up.1
One review suggests that the low SHBG seen with obesity and diabetes is driven mainly by fat in the liver and low-grade inflammation rather than by insulin itself, as was once thought.1 Genetics plays a part too: some common gene variants run with naturally higher or lower levels.1
- Higher SHBG: ageing, hyperthyroidism, chronic infections such as HIV and hepatitis C, advanced liver disease, alcohol, some anticonvulsants1
- Lower SHBG: obesity, type 2 diabetes and insulin resistance, an underactive thyroid, glucocorticoid steroids, androgens including anabolic steroids1
Does SHBG change with age?
Yes, it creeps up. In the Massachusetts Male Aging Study, SHBG rose by about 1.6% per year of age, which helps explain why free testosterone falls faster than total testosterone as men get older.2
In the same data, total testosterone fell by about 0.8% per year of age across the group, while free and albumin-bound testosterone fell by about 2% a year.2 In other words, an older man's total can look reasonable while the share his tissues can easily use is shrinking. There is more on this in how testosterone changes after 40.
Is low SHBG a health warning?
It can be a useful flag for metabolic risk. In a study of men from the Physicians' Health Study II, those with the highest SHBG had about one-tenth the odds of developing type 2 diabetes compared with those with the lowest.3
The researchers also looked at gene variants that nudge SHBG up or down. Those variants shifted diabetes risk in the matching direction, which hints that SHBG may play a causal role rather than being a mere bystander.3 That is a hint, not proof, and nobody should chase an SHBG number for its own sake. But a low result alongside a large waist is a reasonable prompt to check blood sugar and talk to a doctor.
When should SHBG be tested?
Usually alongside total testosterone, when the total is near the lower limit of normal or when a man has a condition likely to alter SHBG.4 The two numbers together allow free testosterone to be calculated.
The Endocrine Society guideline recommends a free testosterone result in exactly these situations, either measured by equilibrium dialysis or estimated with an accurate formula that uses SHBG.4 Reference ranges differ between labs, so compare your number with the range on your own report.
Common questions
Is high SHBG bad for men?
Not in itself, but it can leave less testosterone free to act, so a man with high SHBG may have symptoms despite a normal-looking total. It is also worth asking why it is high, since thyroid and liver problems are among the causes.1
Can you lower SHBG naturally?
The honest answer is to treat the cause rather than the number. If SHBG is raised by an overactive thyroid, heavy drinking or a medicine, dealing with that is what matters.1 Research on supplements for lowering SHBG in people is at an early stage, so check with a doctor before trying one.
What is a normal SHBG level for a man?
Ranges differ between laboratories and shift with age, so there is no single figure to aim for.2 Your doctor will read it alongside your testosterone result rather than on its own.
Read more on this
Sources
- Goldman AL, et al. A reappraisal of testosterone's binding in circulation: physiological and clinical implications. Endocr Rev. 2017;38(4):302-324. PubMed
- Feldman HA, et al. Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts Male Aging Study. J Clin Endocrinol Metab. 2002;87(2):589-598. PubMed
- Ding EL, et al. Sex hormone-binding globulin and risk of type 2 diabetes in women and men. N Engl J Med. 2009;361(12):1152-1163. PubMed
- Bhasin S, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.