Testosterone, Bioavailable (Total, Free, Bioavailable, SHBG) Test
The Bioavailable Testosterone panel measures total testosterone and SHBG and derives the free and bioavailable fractions to assess androgen status with binding-protein context. Including SHBG is central, because it determines how much testosterone is available to tissues and is needed to interpret a total value that misleads when SHBG is abnormal, as in obesity, ageing, thyroid disease, or…
4
Biomarkers
Blood (serum)
Sample
2–4 business days
Turnaround
No
Fasting
What is the Bioavailable Testosterone (with SHBG) test?
This test measures total testosterone and SHBG and derives the free and bioavailable fractions, giving a complete picture of androgen status with the binding-protein context that determines hormone availability. Including SHBG is central to the panel's purpose: because SHBG governs how much testosterone is bound versus available, measuring it allows accurate interpretation of the total value and calculation of the fractions that reflect the hormone reaching tissues. In the diagnostic landscape, total testosterone alone is the first-line measure but is unreliable when SHBG is abnormal — a common situation with obesity, ageing, thyroid or hepatic disease, insulin resistance, and hormonal therapy. By pairing total testosterone with SHBG and deriving free and bioavailable testosterone, this panel resolves that ambiguity in a single test. It is ordered when a thorough assessment of androgen status is needed, particularly where total testosterone is borderline or binding-protein abnormalities are likely. Where the requester's method derives fractions by calculation, the result depends on the equation and albumin assumptions, which differ from the mass-spectrometry-based equilibrium approaches used elsewhere.
What does the Bioavailable Testosterone (with SHBG) test measure?
Testosterone is the principal androgen, produced mainly by the testicular Leydig cells in men and in smaller amounts by the ovaries and adrenal cortex in women, under luteinising hormone and hypothalamic-pituitary-gonadal control. In serum roughly half is bound tightly to sex hormone-binding globulin (SHBG), most of the rest loosely to albumin, with a small free fraction. Total testosterone is the sum of all fractions and shows a morning-peak diurnal rhythm.
Total testosterone is the first-line measure of androgen status but can mislead when SHBG is abnormal. Measuring it together with SHBG allows derivation of the free and bioavailable fractions, which better represent the hormone available to tissues. It anchors the assessment of male hypogonadism and androgen excess in women.
Sex hormone-binding globulin is a hepatically synthesised glycoprotein that binds testosterone and oestradiol with high affinity, regulating their bioavailability. Its concentration is modulated by oestrogens, thyroid hormones, insulin, and hepatic function — rising with ageing, hyperthyroidism, and oestrogen, and falling with obesity, insulin resistance, and androgen excess.
SHBG is the key determinant of how much testosterone is bound versus free, and is essential for interpreting total testosterone and for calculating the free and bioavailable fractions. Abnormal SHBG is precisely the situation in which total testosterone misleads and the derived fractions become necessary.
Free testosterone is the small unbound fraction not attached to SHBG or albumin and is considered the immediately bioavailable, biologically active portion. In this panel it is typically calculated from total testosterone, SHBG, and albumin rather than measured directly, since direct analogue immunoassays for free testosterone are regarded as unreliable.
Free testosterone clarifies androgen status when total testosterone is equivocal or SHBG is altered by obesity, thyroid disease, hepatic disease, ageing, or hormonal therapy. It is particularly useful in diagnosing hypogonadism where total testosterone sits near the lower reference limit.
Bioavailable testosterone comprises the free fraction plus the loosely albumin-bound fraction, which dissociates readily and is accessible to tissues. It is derived by ammonium sulphate precipitation of SHBG-bound hormone or calculated from total testosterone, SHBG, and albumin, and reflects the portion of circulating testosterone realistically available for biological action.
Bioavailable testosterone captures both the free and the readily dissociable albumin-bound fractions, making it valuable where SHBG abnormalities render total testosterone unreliable. It is used alongside free testosterone in the same SHBG-altered or equivocal scenarios to characterise true androgen availability.
Why is the Bioavailable Testosterone (with SHBG) test ordered?
- Investigation of suspected male hypogonadism, particularly where total testosterone sits near the lower reference limit and SHBG status is uncertain
- Assessment of androgen status in men with conditions that alter SHBG — obesity, type 2 diabetes, insulin resistance, thyroid disease, or hepatic disease
- Evaluation of androgen excess in women — hirsutism, acne, or suspected polycystic ovary syndrome — where SHBG is often low and total testosterone misleads
- Monitoring of testosterone replacement therapy where free and bioavailable fractions and SHBG are tracked
- Investigation of erectile dysfunction, reduced libido, or fatigue where androgen deficiency is in the differential
- Assessment in older men where age-related SHBG rise can make total testosterone misleading
- Clarification of a borderline or discordant total testosterone where the SHBG-adjusted fractions are needed
- Evaluation where insulin resistance or metabolic syndrome is suspected to be lowering SHBG and affecting androgen availability
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
2–4 business days
Collection notes
Draw in the morning, ideally before 10:00, to account for diurnal variation when assessing hypogonadism. Document testosterone or anabolic steroid use and high-dose biotin. Defer testing during acute illness where possible, and confirm a low result on a separate morning sample. # REVIEWER FLAG: Confirm whether albumin is measured on the same sample for the calculation.
Specimen requirements
Minimum 1.0 mL serum. Separate serum from cells within 2 hours of collection.
What can affect Bioavailable Testosterone (with SHBG) results?
Testosterone peaks in the morning and declines through the day, so samples for hypogonadism assessment should be drawn in the morning, ideally before 10:00, for valid interpretation.
Obesity, insulin resistance, and androgen excess lower SHBG, while ageing, hyperthyroidism, oestrogen, and hepatic disease raise it. Because the fractions are derived from SHBG, these conditions directly shape the calculated free and bioavailable values.
Calculated free and bioavailable testosterone assume a normal albumin; hypoalbuminaemia or an assumed default albumin can distort the derived fractions. # REVIEWER FLAG: Confirm whether Access Medical Laboratories measures or assumes albumin in its calculation.
Significant illness suppresses the hypothalamic-pituitary-gonadal axis and can alter SHBG, transiently lowering testosterone; assessment should be deferred until recovery where possible.
Androgen supplementation raises measured testosterone and suppresses endogenous production; type, dose, and timing must be documented relative to collection.
High-dose biotin can interfere with streptavidin-biotin steps in testosterone or SHBG immunoassays, distorting measured inputs and therefore the derived fractions. # REVIEWER FLAG: Confirm Access Medical Laboratories assay susceptibility.
What do Bioavailable Testosterone (with SHBG) results mean?
Elevated
Elevated testosterone in men may reflect exogenous androgen use or, rarely, an androgen-secreting tumour; in women it raises concern for polycystic ovary syndrome or adrenal or ovarian androgen excess. A low SHBG with high-normal total testosterone can yield elevated free and bioavailable fractions, clarifying androgen excess that the total value understates.
Normal
Values within age- and sex-specific ranges across the measured and derived parameters indicate adequate androgen status. A normal total with discordant SHBG and derived fractions points to a binding-protein effect and should be interpreted using the free and bioavailable results.
Reduced
Low free and bioavailable testosterone, confirmed on a morning sample, support hypogonadism, and are particularly informative when total testosterone is borderline. A high SHBG can produce low free and bioavailable fractions despite a normal-appearing total, identifying functional deficiency; LH and FSH distinguish primary from secondary causes.
Note: Where the free and bioavailable fractions are calculated, results depend on the equation and on whether albumin is measured or assumed, and are not interchangeable with mass-spectrometry-based equilibrium methods. Total testosterone is unreliable when SHBG is abnormal, which is exactly when the SHBG-adjusted fractions guide interpretation. Morning sampling and confirmation of low results are important, and method differences mean reference ranges are not transferable between laboratories.
Frequently asked questions
It measures total testosterone and SHBG and derives the free and bioavailable fractions. Including SHBG allows accurate interpretation of the total value and calculation of the fractions that reflect the hormone available to tissues.
How to access the Bioavailable Testosterone (with SHBG) test
The Bioavailable Testosterone (with SHBG) reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
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Published: June 3, 2026
Last reviewed: June 3, 2026
- 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)
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. (2018)
- Rosner W, et al. Position statement: Utility, limitations, and pitfalls in measuring testosterone. J Clin Endocrinol Metab. (2007)
- Goldman AL, et al. A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications. Endocr Rev. (2017)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
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