Bioavailable testosterone measures the portion of total testosterone that's actually accessible to tissues, meaning free testosterone plus the weakly-bound fraction that can dissociate from albumin.
Bioavailable T often correlates more strongly with clinical symptoms than total T, especially in older men where SHBG rises and binds more testosterone into unavailable form. Low bio-T with normal total T is a classic aging-male pattern that treatment decisions hinge on.
Optimal Testosterone Bioavailable: 180-362 ng/dL.
Typically re-tested every 10 weeks.
This page explains what the marker measures, how labs produce the number, what typical and optimal ranges look like, and what a high or low result may reflect. It does not tell you what any individual result means for you.
Bioavailable testosterone is the portion of circulating testosterone that is not tightly bound to sex hormone binding globulin (SHBG), which means free testosterone plus the testosterone weakly attached to albumin.
Testosterone does not travel through the bloodstream on its own. Most of it is carried by two proteins. SHBG binds testosterone tightly and holds onto it, so that fraction is largely unavailable to tissues in the short term. Albumin binds testosterone loosely, and because that bond breaks easily as blood passes through capillary beds, the albumin-bound fraction is generally regarded as accessible. A small remainder circulates completely unbound and is reported separately as free testosterone. Bioavailable testosterone adds the free and albumin-bound portions together.
Labs produce the number in one of two ways. Some measure it directly, classically by precipitating the SHBG-bound fraction with ammonium sulfate and assaying what is left. Others calculate it from total testosterone, SHBG and albumin using published binding equations. Both approaches have well-described analytical limitations, and research on testosterone measurement emphasises that method and assay quality materially affect the result reported 1. Work on accurate androgen measurement makes a similar point, noting that mass spectrometry for total testosterone and careful handling of the binding calculation both matter when the goal is a reliable free or bioavailable figure 2. In practice, this means results from two different labs are not always interchangeable.
Related: What Is Testosterone Called on a Blood Test? (Lab Names)
It matters because it tracks the androgen fraction that tissues can actually respond to, which is why it sometimes explains a symptom picture that total testosterone does not.
SHBG is not a fixed quantity. It rises with age, with thyroid hormone excess, with liver disease and with estrogen exposure, and it tends to fall with insulin resistance and higher body fat. When SHBG climbs, total testosterone can sit comfortably inside a reference range while the bioavailable fraction drifts down. When SHBG falls, the reverse happens. That is the practical case for looking at the bioavailable number alongside the total.
A study examining androgen measures and blood pressure reported associations between total, free and bioavailable testosterone, SHBG and hypertension, which supports the idea that androgen status sits inside the broader cardiometabolic picture rather than apart from it 3. Research in men with vasculogenic erectile dysfunction has linked lower testosterone levels with markers of endothelial dysfunction, suggesting that vascular health and androgen status travel together more often than coincidence would predict 4. Studies looking at body composition in men following therapy for rectal cancer have likewise examined testosterone alongside lean mass and fat mass, consistent with the long-standing observation that androgens are associated with how the body distributes tissue 5.
In women, the same logic applies at a much smaller scale. Androgen levels decline across the reproductive years and into the menopausal transition, and clinical writing on androgen status in menopause has used free and bioavailable measures precisely because total testosterone is a poor guide when SHBG is shifting 6.
None of this makes the marker a verdict on your health. It is one input among many, and it is read alongside symptoms and the rest of a panel.
Typical lab reference ranges for bioavailable testosterone differ enormously between men and women, and FitScript's optimal ranges sit inside those intervals.
| Group | Typical lab reference range | FitScript optimal range |
|---|---|---|
| Men | 110 to 575 ng/dL | 180 to 362 ng/dL |
| Women | 0.8 to 15 ng/dL | 3 to 10 ng/dL |
Reference ranges vary by lab, assay method, age and sex, so always read your result against the interval printed on your own report. FitScript's optimal ranges are optimization targets, not diagnostic cut-offs, and a value outside them is not a diagnosis of anything.
There is no age-stratified optimal range for this marker, so none is published here. What can be said is that bioavailable testosterone generally trends downward with age in men, largely because SHBG tends to rise while total testosterone falls, so the usable fraction declines faster than the total does. Some laboratories respond by reporting age-banded reference intervals, which is one more reason two reports can disagree about whether the same number is inside range.
Population differences also exist. A study of postmenopausal women in a large prevention trial reported differences in sex hormone levels, including bioavailable testosterone, across racial and ethnic groups, which suggests a single universal cut-off is an oversimplification 7. Body composition, SHBG, medication use and the time of the blood draw all shift the number as well.
A high bioavailable testosterone result means more unbound and loosely bound androgen is circulating than the reference interval expects, which is interpreted very differently in men and in women.
What people report. In women, higher androgen activity is commonly described alongside acne, unwanted facial or body hair, scalp hair thinning at the crown, and irregular or absent menstrual cycles. In men, a modestly high bioavailable result frequently produces no noticeable symptoms at all, and when symptoms are present they can include acne, oily skin, irritability or disturbed sleep.
Common causes. The two routes to a high number are more testosterone or less binding. Low SHBG raises the bioavailable fraction even when total testosterone is unremarkable, and low SHBG is associated with insulin resistance, higher body fat and some endocrine conditions. In people with polycystic ovary syndrome, elevated free and bioavailable testosterone with low SHBG is a recognised laboratory pattern. Adrenal androgen production, ovarian or testicular sources, and exogenous androgen or DHEA use can all lift the result. Occasionally the explanation is analytical rather than biological, because calculated bioavailable values inherit any error in the total testosterone or SHBG measurement 1.
Over the longer term. In women, sustained androgen excess is associated with metabolic features including insulin resistance, and it is usually investigated rather than watched. A high result is not an emergency and it is not a diagnosis, but a persistently high value, a rapidly rising value, or one accompanied by voice deepening or rapid hair changes is worth discussing with a clinician promptly.
A low bioavailable testosterone result means little usable androgen is reaching tissues, and it can occur even when total testosterone reads as normal.
What people report. Low energy that sleep does not fix, reduced libido, erectile difficulty, flat mood or low motivation, loss of muscle and strength despite training, longer recovery from workouts, and gains in central body fat. In women, the reported picture is less specific and often centres on libido, energy and wellbeing, which is one reason androgen status in women is interpreted cautiously and never from a single value 6.
Common causes. Age-related decline is the most familiar one, with rising SHBG compounding the effect. Excess body fat, poor or short sleep, untreated sleep apnoea, heavy alcohol intake, very low energy availability from aggressive dieting or high training loads, and chronic illness all push the number down. Systemic inflammatory disease shows the same pattern: research in men with ankylosing spondylitis has reported associations between disease activity, testosterone levels and erectile dysfunction 8. Several medication classes are relevant too, and a review of psychotropic medications and male reproduction describes effects on the hormonal axis that can lower testosterone output 9. Pituitary and testicular problems are less common but are part of what a clinician rules in or out.
Over the longer term. Lower androgen availability is associated with less favourable body composition 5, with vascular measures in men with erectile dysfunction 4, and with blood pressure patterns in population data 3. Bone density is also androgen-sensitive over time. These are associations, not a forecast for any one person, and a single suboptimal result is a reason to repeat the test and look at the wider panel rather than to conclude anything.
Plenty, which is why one isolated value is rarely the end of the conversation.
Testing conditions. Testosterone follows a daily rhythm that peaks in the morning in most adults, so draws are usually taken in the early morning, ideally before 10 am, and repeat tests are best done at the same time of day. Acute illness, a recent heavy training session, poor sleep the night before and significant stress can all depress a reading temporarily. Because the bioavailable figure depends on SHBG and albumin, anything that alters those proteins, including liver disease, thyroid status, pregnancy and acute inflammation, moves the result independently of testosterone production.
Medications and supplements. A randomised comparison of metformin against placebo reported effects on sex hormone levels, which indicates that common metabolic medications can register in androgen panels 10. A systematic review and meta-analysis of GLP-1 receptor agonists found changes in testosterone levels, an effect that research suggests may be linked to weight loss as much as to the medication itself 11. Psychotropic medications, opioids, glucocorticoids, estrogen-containing medications and androgen-containing products can all shift either testosterone or SHBG 9. High-dose biotin supplements can interfere with certain immunoassays, so many labs ask that they be paused before testing. Never stop or change a prescribed medication to chase a lab number; that decision belongs to the prescriber.
Habits. Sleep duration and quality, alcohol intake, body fat changes, resistance training, and total energy intake relative to expenditure are all associated with androgen status over weeks to months rather than days. Changes here show up gradually, which is why tracking a trend beats reacting to a single draw.
Method. A calculated bioavailable value and a directly measured one are not the same measurement, and both depend on assay quality 2. Where possible, compare like with like from the same laboratory.
FitScript does not currently test bioavailable testosterone in a panel. If you already have a result from another laboratory, you can upload your labs so that Atlas, FitScript's AI health advisor, can read it alongside the rest of your bloodwork.
That context is the point. A bioavailable testosterone figure read in isolation says very little, while the same figure read next to total testosterone, SHBG, body composition data, metabolic markers and your reported symptoms starts to tell a coherent story. Atlas uses a member's own biology to write your FitScript, and the membership covers 150+ biomarkers tested per year, so trends over time carry more weight than any single draw.
Where a result raises a clinical question, the next step is a conversation with a licensed provider, who evaluates whether any investigation or treatment is appropriate for the individual in front of them. This page is educational and is not medical advice. Discuss your results, and any treatment decision, with a clinician or licensed provider who knows your history.
Bioavailable testosterone is interpreted as part of a group, never alone. Testosterone Total gives the overall circulating amount and is the input most calculated bioavailable values depend on. Testosterone Free isolates the completely unbound fraction and usually moves in the same direction. SHBG is the binding protein that drives the gap between total and bioavailable results, so it often explains an otherwise puzzling pair of numbers. DHEA-S reflects adrenal androgen output, which is particularly relevant when a woman's androgen markers run high. Estradiol matters because testosterone is converted into estradiol, and the balance between them influences symptoms in both sexes. IGF-1 sits alongside these as a broader anabolic and growth axis marker that is often read with androgen status when body composition is the question.
Treat one value as a data point, not a conclusion. Confirm it with a morning repeat draw from the same laboratory, look at it next to total testosterone and SHBG, and note what else was happening in your life that week. Observation, then likely explanation, then a conversation with a clinician: that order keeps you from acting on noise. Research suggests androgen availability is associated with metabolic, vascular and body composition outcomes over time, which is a good reason to track the trend and a poor reason to panic about one number 3 4 5.
Free testosterone is the fraction circulating completely unbound to any protein. Bioavailable testosterone is larger: it adds the testosterone loosely attached to albumin, because that bond breaks easily enough for tissues to access it. Both exclude the fraction held tightly by SHBG.
Typical lab reference ranges are roughly 110 to 575 ng/dL in men and 0.8 to 15 ng/dL in women. FitScript's optimal ranges sit inside those intervals at 180 to 362 ng/dL for men and 3 to 10 ng/dL for women. Ranges vary by lab, assay, age and sex, and optimal ranges are optimization targets rather than diagnostic cut-offs.
Generally yes. In men, total testosterone tends to fall while SHBG tends to rise with age, so the bioavailable fraction often declines faster than the total. In women, androgen levels decline across the reproductive years and into menopause. Some labs publish age-banded reference intervals, which is one reason two reports can classify the same number differently.
It means more usable androgen is circulating than the female reference interval expects. People commonly report acne, unwanted hair growth, scalp hair thinning or irregular cycles. Causes include low SHBG, ovarian or adrenal androgen production, and androgen-containing products. It is a finding to investigate with a clinician, not a diagnosis on its own.
Yes. A modestly elevated result in men frequently causes no noticeable symptoms, and it can reflect low SHBG rather than high production. Because calculated values inherit any error in the total testosterone or SHBG measurement, a repeat morning draw from the same lab is a sensible first step before drawing conclusions.
Usually SHBG. When SHBG is high, more testosterone is locked up and unavailable, so the total can look fine while the usable fraction sits low. SHBG rises with age, thyroid hormone excess, liver disease and estrogen exposure. Reading total testosterone, SHBG and bioavailable testosterone together is what makes the pattern visible.
Most labs ask for an early morning draw, ideally before 10 am, because testosterone follows a daily rhythm. Repeat tests are best taken at the same time of day and at the same laboratory. Acute illness, poor sleep, heavy training the day before and high-dose biotin supplements can all affect the reading, so mention them to whoever ordered the test.
FitScript does not currently test this marker in a panel. If you already have a result from another laboratory, you can upload your labs so Atlas, FitScript's AI health advisor, can read it alongside your other markers and trends.
No. A single value outside a reference or optimal range is a prompt to repeat the test, look at related markers such as total testosterone and SHBG, and consider what else was going on at the time. Any decision about investigation or treatment is made by a licensed provider who evaluates whether it is appropriate for you.