SHBG Blood Test: What It Shows and Why It Matters for Athletes
Many athletes check their total testosterone and stop there. Yet two people with the same total testosterone can have very different amounts of hormone actually available to tissues. The reason is sex hormone-binding globulin, or SHBG. Our editorial team explains what this protein does, how the test is interpreted and why the number deserves attention from anyone monitoring hormonal health.
What SHBG is
Sex hormone-binding globulin is a glycoprotein made mainly in the liver. It circulates in the blood and binds sex steroids, especially testosterone, dihydrotestosterone and estradiol. Hammond (2011) described SHBG as a regulator of how much hormone reaches target tissues and how quickly it is cleared.
Testosterone bound to SHBG is held tightly and is generally considered unavailable to most tissues. Testosterone bound to albumin is held loosely and can detach easily. A small fraction circulates unbound, as free testosterone.
In a typical adult man, a large share of total testosterone is bound to SHBG, most of the remainder to albumin, and only around 2–3% is free. The free and albumin-bound portions together are often called bioavailable testosterone.
Because SHBG holds a big portion of the hormone, changes in SHBG can shift free testosterone even when total testosterone stays the same. That is the core reason the test matters.
How the test is done and interpreted
SHBG is measured in a standard blood sample, usually alongside total testosterone. US laboratories typically report it in nanomoles per liter (nmol/L), and reference ranges vary by lab, sex and age.
The main practical use is calculating free testosterone. Vermeulen et al. (1999) developed a widely used equation that estimates free testosterone from total testosterone, SHBG and albumin. Many US labs report this calculated value automatically.
The Endocrine Society position statement on testosterone measurement (Rosner et al., 2007) warned that direct "analog" free testosterone assays are unreliable and recommended calculated free testosterone or equilibrium dialysis instead. If your report shows free testosterone, it is worth asking which method was used.
Testosterone itself should be measured in the morning, after an overnight fast, and confirmed on a second occasion if low, as recommended in the Endocrine Society guideline on hypogonadism (Bhasin et al., 2018). SHBG is more stable during the day but should be drawn at the same visit.
Causes of high and low SHBG
SHBG responds to many signals, from body weight to liver function and medications. Knowing the main drivers helps make sense of an unexpected result.
| Tends to raise SHBG | Tends to lower SHBG |
|---|---|
| Aging | Obesity, especially abdominal fat |
| Hyperthyroidism | Insulin resistance, type 2 diabetes |
| Estrogens, including oral contraceptives | Hypothyroidism |
| Liver cirrhosis, hepatitis | Androgens and anabolic steroids |
| Some anticonvulsants | Glucocorticoids, nephrotic syndrome |
| Prolonged low energy availability | High-dose growth hormone, acromegaly |
Low SHBG is closely linked to metabolic health. Ding et al. (2009) found in the New England Journal of Medicine that lower SHBG levels were associated with a higher risk of type 2 diabetes in both men and women.
High SHBG in a lean endurance athlete may reflect low energy availability, the condition at the center of relative energy deficiency in sport (RED-S). In that case it can be a clue that training load and food intake are out of balance.
Oral anabolic steroids are known to lower SHBG markedly. Pope et al. (2014) noted this among the endocrine effects of performance-enhancing drugs, along with suppression of the body's own testosterone production.
Why the number matters for athletes
SHBG helps avoid misdiagnosis. A man with symptoms of low testosterone but borderline total testosterone may have high SHBG and genuinely low free testosterone. Conversely, a man with obesity may have low total testosterone that is largely explained by low SHBG, with free testosterone closer to normal.
- Endurance athletes: rising SHBG with falling free testosterone can signal under-fueling.
- Strength athletes gaining fat: falling SHBG may track worsening insulin sensitivity.
- Women: SHBG is part of the workup for hyperandrogenism and polycystic ovary syndrome.
- Users of hormonal drugs: very low SHBG can reflect exogenous androgen exposure and warrants medical review.
Trends over time are more informative than single values. A baseline test during a healthy, stable training period makes later results easier to interpret.
SHBG should always be read together with symptoms, total testosterone, LH, FSH, estradiol and basic metabolic markers. No single number tells the whole story.
Practical steps and what not to do
If your SHBG is outside the reference range, the first step is a discussion with a primary care physician or endocrinologist. They may check thyroid function, liver enzymes, fasting glucose or HbA1c, and review your medications.
Lifestyle factors often move SHBG in the right direction. Weight loss in people with obesity tends to raise SHBG, while restoring adequate energy intake in under-fueled athletes may normalize high values.
Online forums promote supplements said to "lower SHBG," such as boron or certain herbs. Evidence for clinically meaningful effects in healthy people is weak, and chasing a lab number without a medical reason is not a sound strategy.
Above all, a lab value is not a prescription. Starting hormones on the basis of a free testosterone calculation alone, without diagnosis and medical supervision, carries real risks for fertility, blood and cardiovascular health.
Conclusions of the editorial team
SHBG determines how much testosterone is tightly bound and how much is available to tissues, so it is essential for interpreting total testosterone correctly.
Calculated free testosterone using SHBG and albumin is more reliable than direct analog assays, and testing should follow guideline conditions.
High SHBG may point to under-fueling, thyroid or liver issues; low SHBG is linked to insulin resistance, obesity and androgen use.
For more, read our articles on how to interpret LH and FSH, the hematocrit test in athletes, and signs of relative energy deficiency in sport.
References
- Hammond GL. Diverse roles for sex hormone-binding globulin in reproduction. Biol Reprod. 2011;85(3):431–441.
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. J Clin Endocrinol Metab. 1999;84(10):3666–3672.
- Rosner W, Auchus RJ, Azziz R, Sluss PM, Raff H. Position statement: utility, limitations, and pitfalls in measuring testosterone: an Endocrine Society position statement. J Clin Endocrinol Metab. 2007;92(2):405–413.
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744.
- Ding EL, Song Y, Manson JE, 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.
- Pope HG Jr, Wood RI, Rogol A, Nyberg F, Bowers L, Bhasin S. Adverse health consequences of performance-enhancing drugs: an Endocrine Society scientific statement. Endocr Rev. 2014;35(3):341–375.
