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Testosterone, Bioavailable

The part of your testosterone that isn't tightly bound to SHBG, so it is free or loosely held by albumin. It is ordered less often than total testosterone.

AI-verified against cited sources. Awaiting expert reviewAlso called Bio-TLOINC 2990-0

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The usual range for Bioavailable T depends on sex. Choose female or male above to see where you stand.

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What it is

Most testosterone in your blood is attached to proteins, mainly sex hormone-binding globulin (SHBG) and albumin. SHBG holds testosterone tightly. Albumin holds it more loosely. Bioavailable testosterone is the free testosterone plus the part loosely held by albumin, which is everything except the SHBG-bound part. Labs usually calculate it from total testosterone and SHBG, or older methods precipitate the SHBG-bound part out of the sample.

Why it matters

Total testosterone can look normal while the usable part is low, or the other way around, when SHBG is unusually high or low. Bioavailable testosterone is one way to look at that. It is not commonly done (MedlinePlus). Guidelines focus on total testosterone first, then on free testosterone, measured by equilibrium dialysis or estimated with an accurate formula, when total testosterone is near the lower limit or something changes SHBG. Results depend on the method and lab, so compare only with a range for the same method.

If it’s low

A low result can reflect a low total testosterone, or a high SHBG that holds more testosterone tightly. Testosterone levels normally fall with age, and low levels are usual in women, especially after menopause. In men, low levels have many possible causes, and your clinician can look into them. Guidelines advise confirming a low result on another early-morning, fasting sample.

Common causes

  • A lab or method that reads lower than the range you are comparing with
  • A high SHBG level, which holds more testosterone tightly
  • Normal aging
  • In women, usual low levels, especially after menopause
  • In men, problems with the testicles or the pituitary gland, long-lasting liver or kidney disease, or alcohol use disorder
  • Medicines that lower testosterone

What can help

  • Ask whether to repeat the test on another early-morning, fasting sample
  • Ask which method was used, and whether total testosterone, SHBG or free testosterone would add useful information
  • Talk with your clinician about medicines, long-lasting illness and alcohol use, which are linked with low testosterone

If it’s high

A result above your lab's range can reflect a high total testosterone, or a low SHBG that leaves more testosterone unbound. In men it is linked to steroid or testosterone use. In women it is linked to polycystic ovary syndrome (PCOS) and to conditions of the adrenal glands or ovaries. A number that looks high on one method can be usual on another. Your clinician can put it in context.

Common causes

  • A lab or method that reads higher than the range you are comparing with
  • A low SHBG level, which leaves more testosterone available
  • Testosterone therapy, anabolic steroids or other steroid use
  • Polycystic ovary syndrome (PCOS) in women
  • Adrenal conditions, such as congenital adrenal hyperplasia
  • A tumor of the ovaries or testicles (uncommon)

What can help

  • Share every hormone, steroid and supplement you use with your clinician
  • Ask which method your lab used and whether the range on your report is for that method
  • Ask whether total testosterone and SHBG would help put the number in context

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on sex and age. Choose women or men to see the one that applies.

Children and teens 12 ranges

See Bioavailable T by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • Method: calculation and precipitation methods, and immunoassay versus mass spectrometry, give different numbers
  • SHBG and albumin: the result depends on both, and a higher SHBG leaves less testosterone available
  • Time of day: testosterone is highest in the morning, between about 7 and 10 a.m.
  • Age and sex: men's levels fall with age, and women's are much lower, especially after menopause
  • Hormone therapy and steroids, which change total testosterone and SHBG
  • Pregnancy, when calculated testosterone results are less reliable
  • Bioavailable testosterone is calculated from total testosterone, SHBG and the binding constants for SHBG and albumin (ARUP), so any error in those measurements carries into the result.
  • Testosterone immunoassays are imprecise and inaccurate at the low levels seen in children and women. ARUP advises mass-spectrometry testing for them.
  • High-dose biotin supplements can interfere with immunoassays in both directions: results can read falsely high on competitive assays and falsely low on sandwich assays, and falsely high testosterone has been reported in women (Ghazal 2022). Ask your clinician before stopping any supplement.
  • The method changes the number. Calculation and precipitation methods are not identical, so compare only with a range for the same method.
  • Calculated free testosterone agreed with dialysis in every condition studied except pregnancy (Vermeulen 1999). The same binding approach is used for the calculated bioavailable result, so treat results in pregnancy with caution.
  • Testosterone treatment, steroids, estrogen and other hormone medicines change total testosterone and SHBG, and so change this result.

Early morning, between 7 and 10 a.m., when testosterone levels are usually highest (MedlinePlus). The Endocrine Society recommends a fasting morning sample when testosterone is used to look for low testosterone in men. It follows changes in total testosterone and SHBG, so a result reflects your current levels rather than a long-term average.

When to talk to a clinician

A bioavailable testosterone result alone rarely needs urgent action. Talk with a clinician if it is outside the range for your method and you have symptoms. In men, symptoms can include low sex drive, erection problems, trouble having a child, enlarged breasts, loss of muscle or body hair, and thinning bones or anemia without a clear cause. In women, symptoms can include new or fast-growing facial or body hair, hair loss on the scalp, a deeper voice, acne, and irregular or missed periods (MedlinePlus).

Questions you could ask

  • Is bioavailable testosterone useful for me, or would a different test be better?
  • Which method did the lab use, and is the range on my report for that method?
  • Could a medicine or supplement I take be raising it?
  • Would total testosterone and SHBG help explain it?
  • Should we repeat it on an early-morning, fasting sample?
  • Would total testosterone, SHBG or free testosterone add useful information?

Questions people ask

What is the difference between free, bioavailable and total testosterone?

Total testosterone is all of it in your blood. Free testosterone is the small part not attached to any protein. Bioavailable testosterone is free testosterone plus the part loosely held by albumin, so it counts everything except the portion tightly held by SHBG.

Do I need a bioavailable testosterone test?

Often not. It is not commonly done. Most clinicians start with total testosterone. If that is near the edge of the range or SHBG may be off, they may check free testosterone. Ask your clinician which test fits your situation.

How do I convert ng/dL to nmol/L?

Divide ng/dL by 28.84 to get nmol/L. For example, 300 ng/dL is about 10.4 nmol/L. To go the other way, multiply nmol/L by 28.84.

Show the science and sources

Units

  • 1 ng/dL = 1 ng/dL
  • 1 ng/dL = 0.03467 nmol/L
  • 1 ng/dL = 0.01 ng/mL
  • 1 ng/dL = 10 pg/mL

Open the Bioavailable T unit converter

For clinicians and researchers

Bioavailable testosterone is free testosterone plus the albumin-bound (weakly bound) fraction, that is, all testosterone not bound to SHBG. Vermeulen 1999 describes the classic measurement as the fraction of testosterone left in solution after 50% ammonium sulfate precipitation of SHBG (non-SHBG-T), and showed that calculated free testosterone, apparent free testosterone by dialysis and non-SHBG-T all correlate, with calculated and dialysis values nearly identical except in pregnancy. Today labs typically calculate it from total testosterone (immunoassay or mass spectrometry), SHBG and albumin binding constants, as ARUP does. ARUP notes that immunoassay testosterone is imprecise at low concentrations, so mass spectrometry is preferred for women and children, and that no reference intervals exist for males under 14 on its immunoassay-based test. The Endocrine Society (2018) recommends measuring fasting morning total testosterone first and, when it is near the lower limit or SHBG may be altered, obtaining free testosterone by equilibrium dialysis or an accurate formula; it does not set a bioavailable testosterone cutoff. Conversion: ng/dL divided by 28.84 gives nmol/L (molecular weight 288.43).

LOINC 2990-0 (primary), 41018-3, 51005-7, 108779-0. Specimen: serum.

Sources

  1. Testosterone, Bioavailable and Total, Includes Sex Hormone-Binding Globulin (Adult Males or Individuals on Testosterone Hormone Therapy) (test 0070102), ARUP Laboratories
  2. Testosterone, Bioavailable and Total, Includes Sex Hormone-Binding Globulin (Adult Females, Children, or Individuals on Testosterone-Suppressing Hormone Therapy) (test 0081057), ARUP Laboratories
  3. Testosterone Levels Test, MedlinePlus (National Library of Medicine)
  4. A critical evaluation of simple methods for the estimation of free testosterone in serum, Journal of Clinical Endocrinology & Metabolism (1999)
  5. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline, Journal of Clinical Endocrinology & Metabolism (2018)
  6. Hormone Immunoassay Interference: A 2021 Update, Annals of Laboratory Medicine (Ghazal, Brabant, Prie, Piketty) (2022)
  7. Testosterone (PubChem compound 6013), PubChem (National Library of Medicine)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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