A “good” bioavailable testosterone level depends on your sex, age, and the lab running the test, but for adult men most clinical laboratories place the normal range somewhere between roughly 80 and 270 ng/dL, while for a 30-year-old premenopausal woman the reference range is far lower, around 1 to 8 ng/dL. Those numbers, though, come with an unusual amount of fine print. The way bioavailable testosterone is measured or calculated varies enough from method to method that two labs can produce meaningfully different results from the same blood sample, and what counts as “good” also hinges on whether you actually feel healthy at your number.
Why Bioavailable Testosterone Exists as a Concept
Most of the testosterone circulating in your blood is not actually available to your tissues. Roughly two-thirds of it rides through the bloodstream tightly bound to a protein called sex hormone-binding globulin (SHBG), and that fraction is essentially locked up and inactive. Another chunk binds loosely to a more abundant protein called albumin. Only a small slice, usually around 1 to 3 percent in men, floats through the blood completely unattached. Bioavailable testosterone is the sum of the free portion plus the albumin-bound portion. The reasoning is straightforward: because albumin holds testosterone loosely, that testosterone can detach easily when it reaches target tissues, so it behaves more like the free fraction than like the SHBG-bound fraction.
This distinction matters most when total testosterone is borderline or when SHBG levels are unusually high or low. A man whose total testosterone reads as normal might still have low bioavailable testosterone if his SHBG is elevated, because more of that total is being locked away. Conversely, someone with low SHBG might have a total testosterone that looks borderline but a bioavailable level that is perfectly adequate. In those situations, measuring bioavailable or free testosterone can help clarify whether there is a genuine deficiency.1PubMed Central. Testosterone Deficiency – Establishing A Biochemical Diagnosis
Reference Ranges for Men and Women
For adult men, most labs report a bioavailable testosterone reference range in the neighborhood of 80 to 270 ng/dL, though the exact boundaries shift depending on the assay used, the population the lab based its norms on, and the man’s age. Younger men in their 20s and 30s tend to sit in the upper half of that range, while men over 60 often land closer to the lower end. There is no single universally agreed-upon cutoff that separates “good” from “low,” which is one reason clinicians use symptoms alongside lab numbers when making treatment decisions.
For premenopausal women the numbers are much smaller. A study establishing reference ranges in normally cycling healthy women found that the estimated 5th to 95th percentile for bioavailable testosterone at age 30 was about 1.1 to 7.6 ng/dL.2PubMed. Testosterone reference ranges in normally cycling healthy premenopausal women That same study reported parallel ranges for free and total testosterone, and the overlap between those measures underscores how closely they track in women with normal SHBG levels. After menopause, testosterone production drops but SHBG also shifts, so the bioavailable fraction does not always decline in the same predictable way it does in men.
The Measurement Problem
Here is where things get tricky: the number on your lab report depends heavily on how the lab arrived at it. There are two broad approaches. One is a direct assay, which uses a protein called concanavalin A to physically separate the SHBG-bound fraction from the rest, then measures what is left. The other is a calculation that starts with your total testosterone and SHBG levels and plugs them into a mathematical model to estimate the bioavailable fraction.
These two methods do not always agree. Research comparing calculated bioavailable testosterone to the directly assayed version found that the calculated value can be substantially higher than the assayed one. One study reported a mean ratio of calculated-to-assayed bioavailable testosterone of about 2.5, meaning the calculation roughly doubled the direct measurement.3PubMed. Measurement-specific bioavailable testosterone using concanavalin A precipitation: comparison of calculated and assayed bioavailable testosterone A separate study found the discrepancy depended on which mathematical constants were used in the calculation: with standard theoretical constants, calculated values differed from assayed values by more than 30 percent in most samples, but when optimized constants were used, the two methods lined up much better.4PubMed. Serum bioavailable testosterone: assayed or calculated?
The practical takeaway is that comparing a bioavailable testosterone result from one lab or method to a reference range generated by a different method can be misleading. If you are tracking your levels over time, try to use the same lab and the same assay type each time. And if your result seems surprisingly high or low, ask your clinician which method was used before drawing conclusions.
When to Test and Why Timing Matters
Testosterone is not a steady-state hormone. It follows a daily rhythm, peaking in the early morning and declining through the afternoon. Both total and bioavailable testosterone participate in this rhythm. In younger men, the drop from morning peak to afternoon low can be dramatic. One study found a minimum fall of 43 percent in total testosterone from peak to trough, with acrophases for total, bioavailable, and free testosterone occurring between about 7:00 and 7:30 in the morning.5PubMed. Diurnal rhythms of serum total, free and bioavailable testosterone and of SHBG in middle-aged men compared with those in young men
This rhythm has real consequences for diagnosis. In one study, 17 men who had at least one testosterone measurement below 300 ng/dL when tested in the afternoon had normal levels at all three morning visits.6PubMed Central. The effect of diurnal variation on clinical measurement of serum testosterone and other sex hormone levels in men Getting tested after noon can make a perfectly healthy man look deficient on paper. Clinical guidelines for this reason recommend drawing testosterone samples before 10 a.m., and ideally before 9 a.m. if you are under 45. Older men tend to have a slightly blunted rhythm, but even in middle-aged men the morning-afternoon difference is large enough to matter.
What Low Bioavailable Testosterone Feels Like
The symptoms associated with low testosterone are broad and overlap with plenty of other conditions, which is part of why the diagnosis is tricky. The most commonly recognized signs include reduced sex drive, difficulty achieving or maintaining erections, fatigue, mood changes including irritability and low motivation, loss of muscle mass and strength, increased body fat, thinning body hair, and decreased bone density.7PubMed Central. The Relationship between Testosterone Deficiency and Men’s Health
Sexual function tends to be the symptom most directly tied to bioavailable testosterone in research. One study specifically examining bioavailable testosterone and erectile function found that erectile function scores improved significantly as bioavailable testosterone increased, though interestingly, the relationship between bioavailable testosterone and depression scores in the same study was not significant.8PubMed. Bioavailable testosterone with age and erectile dysfunction That does not mean testosterone has nothing to do with mood, but it does suggest that the link between testosterone and sexual symptoms is stronger and more direct than the link to depressive symptoms, which involve many other factors.
Bioavailable Testosterone and Body Composition
One of the clearest arguments for paying attention to bioavailable or free testosterone rather than just total testosterone comes from muscle research. In older men, a study found that free testosterone was positively associated with muscle status, but total testosterone was not.9PubMed Central. Association between total and calculated free serum testosterone and muscle status in young vs. old men In other words, what mattered for whether an older man maintained his muscle mass was not how much testosterone was floating in his blood in total, but how much was actually accessible to tissues.
This pattern shows up in women too. A preprint studying premenopausal women found that bioavailable testosterone was positively related to exercise-induced muscle growth, while total testosterone was not.10bioRxiv. Total testosterone is not associated with muscle mass, function or exercise adaptations in pre-menopausal females The finding reinforces the idea that the bioavailable fraction is the physiologically meaningful number when it comes to muscle, regardless of sex.
Beyond muscle, low testosterone in men is associated with reduced bone mineral density over time, which can progress to osteoporosis.11PubMed Central. Testosterone and Bone Health in Men: A Narrative Review And on the metabolic side, low testosterone levels in men have been linked to obesity, insulin resistance, and unfavorable cholesterol profiles.12PubMed Central. Testosterone and the metabolic syndrome These connections run in both directions: being overweight can lower testosterone, and low testosterone can make it harder to lose weight, creating a cycle that can be tough to break without deliberate intervention.
What Pushes Bioavailable Testosterone Up or Down
Body fat is one of the most modifiable factors. Fat tissue contains an enzyme that converts testosterone to estrogen, so carrying more fat means less testosterone remains in circulation. The good news is that losing weight tends to reverse this. Research has found that dietary changes, particularly lower-carbohydrate or Mediterranean-style eating patterns, have been linked to increased testosterone production in men with obesity, and resistance training also improves hormonal profiles by reducing fat mass.13PubMed Central. Impact of Weight Loss on Testosterone Levels: A Review of BMI and Testosterone Lifting weights seems to be more effective than cardio alone for this purpose, though any exercise that reduces body fat tends to help.
Certain medications can suppress testosterone production substantially. Long-term opioid use is a well-documented cause: research has shown that among men on oral opioids, nearly 90 percent had reduced levels of free testosterone, and a similarly high proportion reported erectile dysfunction and reduced libido.14PubMed Central. Long-term Opioids Linked to Hypogonadism and the Role of Testosterone Supplementation Therapy Corticosteroids, some antidepressants, and certain antifungal drugs can also affect testosterone. If your levels are low and you are taking any of these medications, that connection is worth discussing with your prescriber before assuming you have a primary hormonal problem.
Sleep is another major lever. Testosterone production is tightly linked to sleep quality, with most of the day’s production occurring during sleep. Chronic sleep deprivation, even just a week of five-hour nights, can reduce testosterone significantly. Stress and its associated cortisol elevation also suppress testosterone output, though these effects tend to be reversible once the stressor resolves.
Why Your Number Is Not the Whole Story
Even if your bioavailable testosterone lands squarely in the normal range, that does not guarantee you will feel the effects of it the same way as someone else with the same number. The reason involves the androgen receptor, which is the protein inside cells that testosterone actually binds to in order to exert its effects. A genetic variation in the androgen receptor gene, specifically the length of a repeating segment called the CAG repeat, influences how sensitive your cells are to testosterone. This variation can affect sexual function, fertility, cardiovascular risk, body composition, bone metabolism, and even the risk of certain cancers.15PubMed Central. Influence of CAG Repeat Polymorphism on the Targets of Testosterone Action
The practical consequence is fascinating. A study examining how CAG repeat length interacts with free testosterone and insulin sensitivity found that the CAG length meaningfully changed what testosterone “did” in the body. For men with a CAG length of about 23 (roughly average), changes in testosterone had little effect on insulin sensitivity. For men with longer CAG repeats, meaning less sensitive receptors, higher testosterone improved insulin sensitivity, and the longer the repeat, the bigger the benefit. But for men with shorter CAG repeats, higher testosterone actually had the opposite effect.16PubMed. Androgen receptor CAG repeat length polymorphism modifies the impact of testosterone on insulin sensitivity in men Two men with identical bioavailable testosterone levels can therefore experience genuinely different metabolic outcomes based purely on this one genetic variation.
This is worth knowing if you are comparing your lab results to someone else’s, or if you are puzzled by why your symptoms do not seem to match your number. CAG repeat length is not routinely tested, but it is an important piece of the puzzle that explains why medicine cannot simply define one universal “good” bioavailable testosterone level and apply it to everyone.
Bioavailable Testosterone in Women
Testosterone matters for women too, though the conversation around it gets far less attention. The reference ranges are much lower than in men: as noted earlier, a healthy 30-year-old woman’s bioavailable testosterone typically falls between about 1.1 and 7.6 ng/dL.2PubMed. Testosterone reference ranges in normally cycling healthy premenopausal women Both abnormally high and abnormally low levels can cause problems. Low testosterone in women has been linked to reduced sexual desire and energy, while elevated levels are a hallmark of polycystic ovary syndrome (PCOS).
The relationship between testosterone and sexual function in women with PCOS is not as straightforward as you might expect. One study found that bioavailable testosterone correlated positively with sexual desire and negatively with sexual distress in women with PCOS.17Journal of Pelvic Medicine and Surgery. Sexual dysfunction in women with polycystic ovary syndrome: The effects of testosterone, obesity, and depression But a larger and more recent study with controls found that the association between androgen levels and sexual function in PCOS was weak, with hormonal variables explaining at most about 15 percent of the variation in sexual function, and when the PCOS group was analyzed separately, significant associations disappeared.18PubMed Central. Sexual dysfunction in women with PCOS: a case control study The takeaway is that in women, factors like body image, mood, relationship quality, and other hormonal axes tend to overshadow testosterone’s direct contribution to sexual experience.
Monitoring During Testosterone Therapy
If your bioavailable testosterone is genuinely low and you start replacement therapy, tracking the number becomes even more important. The goal is generally to bring levels into the middle of the normal range, not to push them to the top. One of the key safety concerns during treatment is that testosterone stimulates red blood cell production. When levels climb too high or a person is especially sensitive, the proportion of red blood cells in the blood can rise to problematic levels. Clinical guidelines from the Endocrine Society flag a hematocrit above 50 percent as a relative reason not to start therapy and above 54 percent as a reason to stop therapy, while other professional societies use thresholds between 52 and 55 percent.19PubMed Central. Erythrocytosis Following Testosterone Therapy This is why regular blood work during treatment is standard practice: even if your testosterone levels look perfect on paper, unmonitored therapy can create problems elsewhere.
Beyond blood counts, clinicians also watch for changes in prostate-specific antigen in men, liver function, lipid profiles, and mood. Therapy is not a set-it-and-forget-it intervention. Doses often need adjusting based on follow-up labs, which are typically drawn about six to twelve weeks after starting or changing a dose, with ongoing checks every six to twelve months after that. If you are only getting your testosterone number checked and nothing else, you are not being monitored adequately.
Conditions That Shift SHBG and Distort the Picture
Because bioavailable testosterone is defined by what is left after SHBG claims its share, anything that raises or lowers SHBG will change your bioavailable testosterone level even if your body’s actual testosterone production stays the same. SHBG increases with aging, liver disease, hyperthyroidism, estrogen-containing medications including oral contraceptives, and significant weight loss. SHBG decreases with obesity, hypothyroidism, high insulin levels, and certain androgenic drugs. If you are on an oral estrogen-based contraceptive, for example, your SHBG could be substantially elevated, which would depress your bioavailable testosterone level on a lab test without necessarily meaning your tissues are truly deprived of testosterone. Similarly, an obese man with very low SHBG might show adequate bioavailable testosterone even if his total testosterone is flagged as low.
This is another reason why interpreting bioavailable testosterone in isolation is not ideal. A clinician who sees a low bioavailable testosterone result should also check SHBG, total testosterone, and ideally luteinizing hormone to figure out where in the system the problem lies. Is the body not making enough testosterone? Is SHBG soaking up too much? Is the hypothalamus not signaling the testes properly? These are different problems with different solutions, and a single bioavailable testosterone number cannot distinguish among them on its own.