What is SHBG?

Sex hormone-binding globulin (SHBG) is a protein made in the liver that binds to sex hormones like oestrogen, DHT, and testosterone, regulating their activity in the body. SHBG is often measured to help investigate hormone imbalances. In women, abnormal SHBG levels can contribute to issues like PCOS, irregular periods, and fertility problems. In men, it can lead to symptoms such as low libido and erectile dysfunction. Maintaining optimal SHBG levels is essential for overall health and fertility. 

• What is SHBG?
• SHBG function
• Normal SHBG levels
• Low SHBG
• High SHBG
• SHBG and fertility
   

What is sex hormone-binding globulin (SHBG)

Sex hormone-binding globulin, or SHBG, is a protein made by the liver. As its name suggests, it binds tightly to sex hormones – mainly oestrogen, dihydrotestosterone (DHT), and testosterone. 

It also has a low binding affinity to other oestrogens (oestrone and oestriol), DHEA, and androstenedione. 
 

What does SHBG do?

When sex hormones bind to SHBG, they become biologically inactive and can’t act on the tissues. In this way, SHBG helps regulate their activity. It acts as a form of reservoir for sex hormones, releasing them when their levels drop and binding them when their levels are high. 

SHBG also acts as a transport molecule, carrying the sex hormones to their target cells in an inactive form, preventing them from being broken down en route. 
 

Why do I need an SHBG blood test?

Since SHBG primarily binds testosterone, symptoms of low or high testosterone may warrant an SHBG blood test. 

Common reasons for checking SHBG in women: 

• Investigating polycystic ovary syndrome (PCOS) – PCOS is characterised by high levels of testosterone and insulin resistance, which often causes lower levels of SHBG [1]. 
• Assessing hormone imbalances – An SHBG test can be useful to investigate the causes of symptoms like irregular periods, excessive hair growth, and fertility problems.

When investigating raised androgen levels in women, SHBG can be used to calculate the free androgen index (FAI) to estimate bioavailable testosterone.

Common reasons for checking SHBG in men: 

• Investigating symptoms of low testosterone – Even if testosterone levels are normal, a raised SHBG can decrease its activity and lead to symptoms of deficiency [2]. Symptoms include reduced sex drive, erectile dysfunction, and fertility problems. 
   

What is a normal SHBG level for my age?

SHBG levels tend to increase with age in men as their testosterone levels drop. In women, SHBG levels can vary over time, especially in response to declining oestrogen during menopause, or if taking hormone replacement therapy (HRT). 

Normal SHBG ranges vary according to the lab. The ranges below are given as a guide only. 

Age	Men (nmol/L)	Women (nmol/L)
20–49	16.5–55.9	24.6–122
50+	19.3–76.4	17.3–125

 

SHBG levels in pregnancy

SHBG levels increase during pregnancy due to increased oestrogen production [3]. Levels usually return to normal soon after birth. 

Gestation	SHBG (nmol/L)
1st trimester	39–131
2nd trimester	214–717
3rd trimester	216–724

 

What happens if SHBG is too low?

When SHBG levels are too low, it can lead to an excess of free testosterone and oestrogen which can oversaturate receptors and cause problems. Studies have shown that low SHBG levels are linked to some conditions. 

Low SHBG levels are linked to [4]: 

• Insulin resistance and diabetes
• Obesity
• Heart disease
• PCOS
• Arthritis
• Liver disease
• Some cancers

It’s not clear exactly why, but it’s likely to do with the direct effect of SHBG on tissues as well as an imbalance of the sex hormones. Some conditions, like obesity, have a bidirectional relationship, i.e. they are both a cause and consequence of low SHBG.
 

Causes of low SHBG

Genetic makeup accounts for about 40–60% of the variation in SHBG levels among individuals [5,6]. However, some conditions and medications can cause a low SHBG result. 

Causes of low SHBG include: 

• Obesity – Fat tissue produces hormones, like leptin, and inflammatory markers that can reduce SHBG production.
• Insulin resistance and metabolic syndrome – When tissues become resistant to insulin, the pancreas makes more. High levels of insulin decrease SHBG production. 
• Hypothyroidism (underactive thyroid) – Thyroid hormones indirectly increase SHBG by increasing the production of SHBG in the liver. Therefore, an underactive thyroid and smaller amounts of thyroid hormones can cause lower SHBG levels. 
• Polycystic ovary syndrome (PCOS) – Women with PCOS often have lower SHBG levels due to insulin resistance and higher androgen (male hormone) levels. 
• TRT and anabolic steroids – Higher levels of testosterone in the body typically signal the liver to produce less SHBG. 
• Acromegaly – High levels of growth hormone (GH), as seen in conditions like acromegaly, inhibit SHBG production. 

Taking biotin supplements, particularly high doses can interfere with the SHBG assay and cause a falsely low result. If you take biotin supplements, it’s best to stop these at least a couple of days (or longer for very high doses) before your test, after discussing this with your doctor. 
 

How to increase SHBG levels

If your SHBG levels are low, it’s important to first rule out and treat possible causes. For example, if you’re overweight, losing weight is likely to help. Or if you have an underactive thyroid, treating the condition with medication will likely improve your SHBG levels. However, some lifestyle changes can also be effective. 

Ways to naturally increase SHBG levels: 

• Get regular exercise – A recent systematic review found that regular exercise (resistance training and particularly high-intensity training) increases basal levels of SHBG in both men and women [7].
• Lose weight if you’re overweight – Sustained weight loss is shown to increase SHBG levels [8,9].
• Increase fibre intake – Some research has shown that people with higher fibre intake were more likely to have higher SHBG levels, though some older studies disagree [10].
   

What happens if SHBG is too high?

A higher SHBG is protective against conditions like diabetes and heart disease [11–13]. However, if SHBG levels are excessively high, it binds to a larger proportion of sex hormones, resulting in lower levels of bioavailable testosterone and oestrogen. This can lead to symptoms of a deficiency, even if total testosterone and oestrogen levels are normal. 

High SHBG has also been linked to: 

• Lower bone mineral density and increased risk of fractures [14]
• A higher risk of stomach cancer [15]
• Dementia [16]

Based on the study designs, we don’t know whether higher SHBG levels cause stomach cancer or dementia, only that there is a correlation. It could be that SHBG increases secondary to these conditions or other confounding factors. For example, SHBG levels tend to be higher in older men, and age is a risk factor for both stomach cancer and dementia. 

So, don’t panic if your result is raised, however, it is important to rule out potential causes.
 

Causes of high SHBG

High levels of SHBG often result in lower levels of bioavailable testosterone and oestradiol which can affect overall health. 

Common causes of high SHBG include:

• Male ageing – SHBG levels naturally increase with age in men. One study showed that levels may be up to 40% higher in men in older age categories [17].
• Liver disease – Opposite to what you’d expect, cirrhosis can lead to an overproduction of SHBG, where the hormone is made. It’s thought this could be a compensatory response to lower testosterone levels [18]. 
• Hyperthyroidism – Higher levels of thyroid hormones inhibit SHBG production in the liver. 
• Oral contraceptives – Oral contraceptives, particularly those with higher doses of oestrogen, are linked to higher SHBG levels [19].
• Hormone replacement therapy (HRT) – HRT, for the same reasons as oral contraceptives, can increase SHBG levels. 
• Antiepileptic medications – Medications for epilepsy can increase SHBG levels over time which can lead to lower levels of bioavailable testosterone and oestradiol [20].
• Pregnancy – SHBG levels are naturally higher during pregnancy, especially in the second and third trimesters. 
• Anorexia – Severe calorie restriction and malnutrition can increase SHBG. 
• A lack of physical activity – Regular exercise has been shown to lower SHBG levels, while inactivity can have the opposite effect. 
   

How to lower SHBG

If your SHBG levels are high, it’s important to identify and address any underlying conditions that may be contributing, such as hyperthyroidism. However, studies have shown that some lifestyle factors, such as diet, may have a weak effect on lowering SHBG levels. 

Ways to naturally lower SHBG: 

• Eat more protein – Particularly for ageing men, eating plenty of protein is likely to help keep SHBG levels low, possibly through its effect on insulin [10].
• Eat a high-fibre diet – In women, a high-fibre diet has been shown to decrease SHBG levels [10].

It’s also important to maintain a healthy weight, avoid excess alcohol, limit your intake of caffeine and sugar, and make sure you’re getting enough micronutrients (vitamin D, zinc, magnesium, etc.). These are important for overall hormone balance.
 

Can SHBG levels affect fertility?

SHBG levels can affect fertility by disrupting the balance of bioavailable sex hormones, like testosterone and oestradiol. 

High levels of SHBG can lower levels of free testosterone, which may affect sperm production and libido in men. In women, raised SHBG can reduce the amount of free oestrogen impacting the menstrual cycle and ovulation, which are critical for fertility. 

On the other hand, low levels of SHBG can increase availability of testosterone and oestrogen, potentially leading to hormone imbalances that might also impair reproduction. This is seen in conditions like PCOS. 

Therefore, maintaining optimal SHBG levels is essential for fertility in both men and women.
 

What to do if your SHBG levels are abnormal

If your SHBG levels are abnormal, it’s best to speak to your doctor. You may need other tests to rule out any underlying cause. Our doctors will advise on what to do next if one of your results is outside the normal range. In the meantime, your lifestyle choices can have a big impact on your hormone levels. Try these three expert ways to balance your hormones.
 

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References

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2. Carnegie C. Diagnosis of Hypogonadism: Clinical Assessments and Laboratory Tests. Rev Urol. 2004;6: S3–S8.
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