Key takeaways:
- Sex hormone binding globulin, or SHBG, is a protein made mostly by the liver that binds to testosterone in the blood, and only the small unbound fraction, called free testosterone, is actually available for your cells to use.
- A 2021 analysis of national U.S. health data found that clinically low SHBG affects roughly 3.3% of adult men and is most strongly predicted by a higher body mass index, followed by diabetes, smoking, and several chronic conditions (Wang, Journal of Clinical Endocrinology and Metabolism, 2021).
- A related version of this same protein, called androgen binding protein, is made directly inside the testes by cells called Sertoli cells and concentrates testosterone exactly where sperm are produced, which is part of why SHBG genetics have been linked to sperm concentration in men with otherwise normal semen parameters (Lazaros et al., Andrologia, 2008).
Most men who get a hormone panel back read the testosterone number first and skip straight past SHBG, treating it like a technical footnote the lab includes for its own reasons. That is a mistake. SHBG is very often the reason two men can have the exact same total testosterone result on paper and a completely different amount of testosterone actually available to their bodies, and by extension, to their testes.
Think of testosterone traveling through your bloodstream the way a package travels through a delivery network. Some of it is sealed in a locked container that cannot be opened in transit, some of it is loosely wrapped and can be accessed with a bit of effort, and only a small amount is sitting out in the open, ready to be picked up and used the moment it arrives. SHBG is the locked container. It is not a hormone itself. It is a transport protein, and how much of it you have determines how much of your testosterone is genuinely working for you versus how much is just along for the ride.
This matters for fertility specifically because a version of this same protein is made inside the testes themselves, where it plays a direct role in keeping local testosterone concentrated enough to support sperm production. This article explains what SHBG actually does, what a high or low result means, and where the evidence is solid versus where it is still thin.
What Does SHBG Actually Do?
SHBG is a protein, made mostly by the liver, that grabs onto testosterone as it circulates in the blood and holds it in a bound, inactive form. It does the same thing to a lesser degree with estradiol, the form of estrogen men also produce. Once testosterone is bound to SHBG, it cannot enter cells or activate androgen receptors, the docking sites inside cells that testosterone has to lock onto to actually trigger an effect. It is effectively taken out of circulation until it is released again.
In a healthy adult man, roughly 44% of total testosterone is tightly bound to SHBG, a little over half is loosely bound to a more abundant protein called albumin, and only about 2% is completely free and immediately available to tissues (Dunn, Nisula, and Rodbard, Journal of Clinical Endocrinology and Metabolism, 1981). The free fraction plus the loosely albumin-bound fraction together are called bioavailable testosterone, since albumin's grip is weak enough that testosterone can detach from it easily when a cell needs it. SHBG's grip is much tighter, which is why the SHBG-bound fraction is generally treated as unavailable for practical purposes. This is why two men can have identical total testosterone results and very different amounts of usable testosterone: the man with higher SHBG has more of his testosterone locked away, and the man with lower SHBG has more of it free and bioavailable.
Fertility enters the picture through a second, less well known role. The liver is not the only place this protein is made. Sertoli cells, the supporting cells inside the testes that nurture developing sperm, produce their own local version of it called androgen binding protein. It is the same gene and a very similar molecule, but instead of circulating in blood, it is secreted directly into the fluid of the seminiferous tubules, where sperm are actually produced. There, its job is to bind testosterone and keep it concentrated inside the tubule rather than letting it diffuse away, which is one of the mechanisms that helps intratesticular testosterone stay far higher than the testosterone level measured in a standard blood draw, a difference on the order of 50 to 100 times. That local concentration is not a curiosity. It is a requirement, since the genes that drive sperm maturation and DNA packaging need that much higher local dose of testosterone acting on the androgen receptor inside Sertoli cells to function properly (Gill-Sharma, The Open Biochemistry Journal, 2018).
What Does a Normal SHBG Level Mean?
Most labs report a normal adult male range somewhere around 10 to 57 nmol/L, though the exact cutoffs shift slightly depending on the assay a given lab uses, which is part of why SHBG is more useful read as a pattern alongside other hormones than as a single number compared to a chart. A normal SHBG result means your liver's transport-protein output is behaving typically for your age and health status, and that a normal-looking total testosterone result is likely giving you a reasonably accurate picture of your usable testosterone as well.
Where SHBG becomes genuinely useful is at the edges. When SHBG is unusually high or low, total testosterone alone stops being a reliable stand-in for how much testosterone your body can actually use, and a calculated or directly measured free testosterone becomes the more meaningful number. The most widely used method for this, developed by Vermeulen and colleagues, calculates free testosterone from total testosterone, SHBG, and albumin using a formula grounded in how strongly each protein binds the hormone, and it has been shown to correlate closely with free testosterone measured by the more expensive, more technically demanding direct method called equilibrium dialysis (Vermeulen, Verdonck, and Kaufman, Journal of Clinical Endocrinology and Metabolism, 1999). In plain terms, once your doctor has your total testosterone and your SHBG, calculating a meaningful free testosterone number does not require an extra blood draw. It requires doing the math with the numbers already in hand.
What Does High SHBG Mean for Your Fertility?
High SHBG means more of your circulating testosterone is bound and unavailable, even if the total testosterone number on your report looks normal or high. The most common driver is simply age. In a large longitudinal study following more than 1,700 community-dwelling men over roughly five years, SHBG rose steadily with age and tracked upward alongside total testosterone, while higher triglycerides and more abdominal fat were independently associated with lower SHBG in the same cohort (Gyawali et al., PLoS One, 2018). An overactive thyroid, called hyperthyroidism, is another recognized cause: excess thyroid hormone raises SHBG, which can push total testosterone up on paper while the free testosterone actually available to tissues stays flat or even falls, a pattern our TSH article covers from the thyroid side (Sengupta et al., Life, 2022). Liver disease and certain anticonvulsant medications are less common but recognized causes as well, since the liver's overall protein output and metabolism both influence how much SHBG ends up in circulation.
The practical trap with high SHBG is a falsely reassuring total testosterone result. A man with high SHBG can have a total testosterone reading comfortably inside the normal range while his actual free testosterone, the fraction his testes and other tissues can use, sits closer to a deficient level. This is exactly the scenario calculated free testosterone is built to catch, and it is worth specifically requesting when a total testosterone result does not match how you feel, or does not match other findings like low sperm concentration on a semen analysis.
What Does Low SHBG Mean for Your Fertility?
Low SHBG means less of your testosterone is locked away, which sounds like it should be good news, and sometimes is, but it also comes with its own set of causes and complications. The strongest and most consistent driver is metabolic: obesity and insulin resistance both suppress SHBG production by the liver. A study of 150 obese men found that 52.0% had a testosterone deficiency based on total testosterone alone, but that figure dropped to 17.6% once calculated free testosterone was used instead, and lower SHBG together with higher insulin resistance were each independently correlated with lower free testosterone, meaning the two problems compound each other rather than one simply explaining the other (Souteiro et al., Andrologia, 2018). A 2021 analysis of a large, nationally representative U.S. health survey found that clinically low SHBG affects an estimated 3.3% of adult men, with a higher body mass index as the single strongest predictor, followed by diabetes, chronic obstructive pulmonary disease, coronary heart disease, smoking, heavy alcohol use, and a prior heart attack (Wang, Journal of Clinical Endocrinology and Metabolism, 2021). An underactive thyroid also lowers SHBG, the mirror image of the hyperthyroid effect described above (Sengupta et al., Life, 2022). Taking exogenous testosterone or anabolic steroids suppresses SHBG production as well, which is one of several hormonal disruptions layered on top of the shutdown of natural sperm production that our TRT-induced infertility article covers in depth.
A small number of men have genuinely low SHBG for genetic reasons rather than a metabolic or lifestyle cause. A study of more than 1,500 fertile and infertile men found that specific SHBG gene variants reliably predicted lower circulating SHBG and lower total testosterone, but also found that free testosterone stayed comparatively stable across these genetic differences, suggesting the body partially compensates when SHBG itself shifts for genetic reasons (Grigorova et al., Journal of the Endocrine Society, 2017). At the extreme end of that spectrum, a 2025 case report described a man with a rare genetic SHBG deficiency severe enough to make his calculated free testosterone unreliable: a direct, more technically demanding measurement showed his free testosterone was genuinely low, yet he had no clinical symptoms of low testosterone and normal LH, illustrating that when SHBG is extremely low, the standard calculation can stop matching reality in either direction (Manz et al., JCEM Case Reports, 2025). That is a single case, not a population finding, and most men with low SHBG have an ordinary metabolic explanation rather than a rare genetic one, but it is a useful reminder that at the far edges of any range, a formula is not a substitute for a direct measurement.
How Should You Read SHBG Alongside Other Hormones?
SHBG rarely tells its own complete story. It is most useful read next to total testosterone, calculated free testosterone, and LH together. A normal total testosterone with high SHBG and a low calculated free testosterone points toward a real, functional testosterone shortfall that a total-testosterone-only test would miss entirely. A low total testosterone with low SHBG can look alarming on the total number alone, but the calculated free testosterone in that scenario is sometimes closer to normal than the headline number suggests, since less of what is circulating is locked away. Layering in LH sharpens the picture further: a low calculated free testosterone alongside a high LH points toward a problem originating in the testes themselves, the same pattern our article on high LH and low testosterone walks through, while a low calculated free testosterone with a normal or low LH points further upstream, toward the pituitary or hypothalamus, which our LH article and FSH article cover in more depth.
SHBG's local cousin, androgen binding protein, also connects it to a semen analysis result directly rather than only to a blood hormone panel. Because androgen binding protein concentrates testosterone inside the seminiferous tubules where sperm develop, it is reasonable to ask whether the same genes that shape circulating SHBG also shape sperm production, and there is direct if modest evidence that they do. A study of 114 men, including 85 with normal sperm counts and 29 with a low sperm count, found that men carrying a shorter version of the SHBG gene's repeat region had meaningfully higher sperm concentration than men with the longer version (Lazaros et al., Andrologia, 2008). It is a small, single study from 2008 and has not been widely replicated at a larger scale since, so we would not treat it as a settled fact, but it fits the underlying biology: a protein whose entire job is concentrating testosterone where sperm are made is a plausible place for genetic variation to show up in sperm counts.
What We're More Skeptical About
SHBG genetics research is a good example of a finding that is real but easy to overstate. The 2017 study of more than 1,500 fertile and infertile men found clear genetic effects on SHBG and total testosterone, but it also found that free testosterone stayed comparatively stable across those same genetic variants, which suggests the body has some capacity to compensate when SHBG shifts for genetic reasons (Grigorova et al., Journal of the Endocrine Society, 2017). That is a meaningfully different, more modest claim than saying SHBG genetics determine fertility outcomes, and the 2008 sperm concentration finding described above comes from a single, small, dated study that deserves to be read as suggestive rather than definitive.
We are also skeptical of the growing number of supplements and wellness products marketed to "optimize" or "balance" SHBG for fertility or testosterone purposes. Unlike CoQ10 for DNA fragmentation or L-carnitine for motility, where a real if imperfect evidence base exists, we are not aware of a supplement with a rigorous clinical trial showing it safely and predictably changes SHBG in a way that improves fertility outcomes. The interventions with actual evidence behind them, weight loss and improved insulin sensitivity for low SHBG, treating a genuine thyroid disorder for either direction, are medical and lifestyle changes, not something in a supplement bottle. And as the 2025 case report above shows, at the extreme ends of the SHBG range, even the standard calculation can become unreliable, which is a reason to lean on a full pattern of results and a clinician's judgment rather than fixating on the SHBG number in isolation.
What Can Be Done
If your SHBG comes back outside the typical range, the first useful step is making sure a calculated free testosterone was generated from your total testosterone, SHBG, and albumin, since that number, not the total testosterone alone, is what actually reflects how much testosterone your body has to work with. If it was not calculated automatically, this is a reasonable thing to ask your doctor or lab for directly.
From there, what to do next depends on which direction your SHBG is off and why. If low SHBG is paired with a higher body mass index, elevated blood sugar, or other markers of insulin resistance, the interventions with the best evidence are weight loss and improved insulin sensitivity, which several of the studies above link directly to SHBG normalizing over time. If either a high or low SHBG shows up alongside an abnormal TSH, correcting the underlying thyroid disorder is the medically appropriate path, not a supplement. If you are currently using testosterone therapy or anabolic steroids, low SHBG is an expected part of that picture, alongside the broader fertility risks our TRT-induced infertility article covers, and the relevant conversation is about the testosterone use itself rather than the SHBG number in isolation. In every case, give any intervention at least 12 weeks before retesting, since that is roughly how long a full cycle of sperm production takes, and measure again with the same lab and the same assay so any comparison is meaningful.
Our Take
We know SHBG is not a side note. It is the variable that determines whether a given total testosterone number reflects a man's actual hormonal and reproductive reality or obscures it, in either direction. We know its testicular cousin, androgen binding protein, plays a direct, mechanistically important role in keeping local testosterone high enough for sperm production, and that genetic variation in this system has a real, if modest, link to sperm concentration.
What we do not know, and will not pretend otherwise, is how often correcting an abnormal SHBG on its own, separate from treating whatever is driving it, meaningfully changes a man's fertility outcome. SHBG is best understood as a lens that clarifies what your testosterone number actually means, not as an independent target to chase. The practical path is to get it measured alongside total testosterone, LH, and a full semen analysis, use it to generate a genuine free testosterone number, and address whatever underlying driver, whether metabolic, thyroid, or otherwise, the full pattern points to.
FAQ
What does a high SHBG level mean?
A high SHBG level means more of your circulating testosterone is bound and unavailable to your tissues, which can make total testosterone look normal or even high while the free testosterone your body can actually use is lower than the total number suggests. Common causes include normal aging and an overactive thyroid (Gyawali et al., PLoS One, 2018; Sengupta et al., Life, 2022).
What does a low SHBG level mean?
A low SHBG level means less of your testosterone is bound, and it is most commonly linked to obesity, insulin resistance, an underactive thyroid, or testosterone or anabolic steroid use, with a nationally representative U.S. study finding a higher body mass index as the strongest predictor (Wang, Journal of Clinical Endocrinology and Metabolism, 2021).
Should I get my SHBG tested if my testosterone is already normal?
It is worth requesting alongside testosterone, especially if you have symptoms that do not match a normal testosterone result, since SHBG lets your doctor calculate a genuine free testosterone number rather than relying on the total testosterone figure alone.
Can losing weight improve my SHBG and testosterone levels?
For men whose low SHBG is driven by obesity and insulin resistance, weight loss and improved insulin sensitivity are the interventions with the strongest evidence, since both factors have been independently linked to lower SHBG and lower free testosterone (Souteiro et al., Andrologia, 2018).
Does SHBG directly affect sperm count?
Not in the way blood SHBG is usually discussed, but a closely related protein made inside the testes, called androgen binding protein, concentrates testosterone where sperm are produced, and genetic variation in the SHBG gene has been linked to sperm concentration in a small study of 114 men, a finding that is suggestive but has not been widely replicated at a larger scale (Lazaros et al., Andrologia, 2008).
References
- Dunn JF, Nisula BC, Rodbard D. Transport of steroid hormones: binding of 21 endogenous steroids to both testosterone-binding globulin and corticosteroid-binding globulin in human plasma. Journal of Clinical Endocrinology and Metabolism. 1981;53(1):58-68. https://pubmed.ncbi.nlm.nih.gov/7195404/
- Vermeulen A, Verdonck L, Kaufman JM. A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology and Metabolism. 1999;84(10):3666-3672. https://doi.org/10.1210/jcem.84.10.6079
- Gill-Sharma MK. Testosterone Retention Mechanism in Sertoli Cells: A Biochemical Perspective. The Open Biochemistry Journal. 2018;12:103-112. https://pmc.ncbi.nlm.nih.gov/articles/PMC6048825/
- Wang Y. Definition, Prevalence, and Risk Factors of Low Sex Hormone-Binding Globulin in US Adults. Journal of Clinical Endocrinology and Metabolism. 2021;106(10):e3946-e3956. https://pmc.ncbi.nlm.nih.gov/articles/PMC8571812/
- Gyawali P, Martin SA, Heilbronn LK, et al. Cross-sectional and longitudinal determinants of serum sex hormone binding globulin (SHBG) in a cohort of community-dwelling men. PLoS One. 2018;13(7):e0200078. https://pmc.ncbi.nlm.nih.gov/articles/PMC6040731/
- Souteiro P, Costa D, Amaro C, et al. Insulin resistance and sex hormone-binding globulin are independently correlated with low free testosterone levels in obese males. Andrologia. 2018;50(7):e13035. https://pubmed.ncbi.nlm.nih.gov/29744905/
- Lazaros L, Xita N, Kaponis A, Zikopoulos K, Sofikitis N, Georgiou I. Evidence for association of sex hormone-binding globulin and androgen receptor genes with semen quality. Andrologia. 2008;40(3):186-191. https://pubmed.ncbi.nlm.nih.gov/18477206/
- Grigorova M, Punab M, Poolamets O, Adler M, Vihljajev V, Laan M. Genetics of Sex Hormone-Binding Globulin and Testosterone Levels in Fertile and Infertile Men of Reproductive Age. Journal of the Endocrine Society. 2017;1(6):560-576. https://academic.oup.com/jes/article/1/6/560/3574887
- Manz XD, Vos MJ, Booij RS, Sanson R, Jacobs LHJ. Genetic SHBG Deficiency Unmasks the Discrepancy Between Biochemical and Clinical Hypogonadism. JCEM Case Reports. 2025;3(12):luaf273. https://pmc.ncbi.nlm.nih.gov/articles/PMC12635475/
- Sengupta P, Dutta S, Karkada IR, Chinni SV. Endocrinopathies and Male Infertility. Life. 2022;12(1):10. https://pmc.ncbi.nlm.nih.gov/articles/PMC8779600/
This is Part 7 of the SwimScore Hormone Series, completing our tour through the core hormone panel: FSH, LH, testosterone, prolactin, estradiol, TSH, and SHBG.
See the earlier hormone articles on FSH, LH, testosterone, prolactin, estradiol, and TSH.
Want a genuine free testosterone picture alongside a full semen analysis? SwimScore Complete measures your full hormone panel, including testosterone and SHBG, alongside sperm concentration, motility, morphology, and DNA fragmentation, from home, processed in CLIA-certified labs.
Fertility clinics and urology practices: if a patient's total testosterone does not match their symptoms or semen analysis, see how SwimScore can add a same-visit SHBG and free testosterone picture on our clinic page.
This article is for general education and is not medical advice. An abnormal SHBG or testosterone result should be reviewed with a physician or endocrinologist who can interpret it alongside your full hormone panel and history.
SwimScore uses CLIA-certified labs for all semen analysis and hormone testing, assessed against WHO 6th Edition clinical thresholds.
