Bioavailable Testosterone Test: What Your Tissues Can Use

What is bioavailable testosterone?

Bioavailable testosterone is the share of your testosterone that is either completely free or loosely bound to albumin, the most common protein in blood. Both forms can reach tissues such as muscle, bone and brain. It excludes testosterone bound tightly to SHBG, which is largely unavailable. It is calculated from your total testosterone, SHBG and albumin.

Testosterone travels in three states. A small share is free testosterone, unbound and active. A larger share sits loosely on albumin, and because albumin's grip is weak, it can come off as blood passes through tissues. The rest is held tightly by sex hormone-binding globulin (SHBG) and mostly stays out of action.

The formula uses the same inputs as calculated free testosterone, so the two usually move together. Bioavailable is the wider measure, so it is reported in nmol/L while free testosterone uses pmol/L.

What does a bioavailable testosterone test show?

A bioavailable testosterone test shows how much testosterone your tissues can use, and it matters most when SHBG is pulling your total testosterone result in a misleading direction. It strips out the SHBG-bound share.

Two common situations:

• SHBG is high. SHBG rises with age, an overactive thyroid, liver disease, low body weight, under-eating, and oestrogen from the pill, oral HRT or pregnancy. Total testosterone can look normal while bioavailable testosterone is low, because more is locked away.
• SHBG is low. Carrying extra weight, insulin resistance, type 2 diabetes and an underactive thyroid lower SHBG. In men this often makes total testosterone look low while bioavailable testosterone can be adequate. In women, low SHBG can push bioavailable testosterone up and drive acne or excess hair growth.

Because SHBG slowly rises over the decades, bioavailable testosterone falls faster with age in men than total testosterone does. Normal does not always mean optimal. A man in his 40s with low-normal bioavailable testosterone and rising SHBG is worth tracking, even if nothing needs treating today.

Bioavailable testosterone normal range

For adult men, bioavailable testosterone is usually about 2.5 to 12.0 nmol/L. For women, it is much lower, about 0.1 to 1.3 nmol/L when tested on day 2 to 5 of the cycle.

Because this is a calculated marker, ranges vary more between labs than for most tests, so read your result against the range on your report. Optimal ranges are what we aim for to support long-term health, not diagnostic cut-offs.

What does high bioavailable testosterone mean?

High bioavailable testosterone usually means either more testosterone is being made or taken, or SHBG is low so more is available. In women, the most common cause is polycystic ovary syndrome (PCOS).

• PCOS in women. The insulin resistance that often comes with PCOS lowers SHBG, so more of the extra testosterone stays available.
• Low SHBG, in men and women, most often from carrying extra weight or insulin resistance.
• Testosterone therapy, anabolic steroids, DHEA or prohormone supplements.
• Less commonly in women, non-classic congenital adrenal hyperplasia or, rarely, an androgen-producing tumour.

Men who aren't using testosterone or steroids rarely need to worry about a high-normal result. Women with a high result and symptoms such as acne, excess hair or irregular periods should see a GP.

What does low bioavailable testosterone mean?

Low bioavailable testosterone means less testosterone is reaching your tissues, either because you are making less or because high SHBG is locking more of it away. In men, a low result needs repeating with an early morning test before it means much.

Common reasons in men:

• The everyday causes of low testosterone: an afternoon test, extra weight and insulin resistance, poor sleep, heavy alcohol, recent illness or a run of hard training, and medicines such as opioids. Our free testosterone page goes through these.
• High SHBG, from the causes listed above. This is what the marker is best at catching, because total testosterone can look normal.

Less often, the problem is in the testes or the pituitary gland's signal to them. A GP will check LH, FSH and prolactin to tell these apart.

In women, low bioavailable testosterone is very common on the combined pill or oral HRT, because oestrogen taken by mouth raises SHBG. It is rarely acted on by itself.

What can affect your bioavailable testosterone result

Anything that changes testosterone, SHBG or albumin will shift this calculated result. Timing follows the same rules as free testosterone: men test early in the morning, ideally before 10am, and women with periods test on day 2 to 5.

• Oestrogen taken by mouth (the combined pill or oral HRT) raises SHBG and pulls bioavailable testosterone down.
• Pregnancy raises SHBG sharply, so results are hard to interpret.
• Albumin. Low albumin from illness, liver or kidney disease, or poor nutrition can make the calculated result less reliable. Some labs assume a standard albumin level if yours isn't measured.
• Supplements. DHEA and testosterone products raise levels, and high-dose biotin can affect some hormone assays, so tell your practitioner what you take.

Do you need to fast? No. Men should still test before breakfast, and with your Baseline you'll be fasting anyway.

How bioavailable testosterone fits with your other results

Bioavailable testosterone is only as meaningful as the SHBG and albumin results behind it.

How to support healthy bioavailable testosterone levels

The most useful levers are the ones that move SHBG and insulin towards a healthy range. If the cause sits in the testes or pituitary, treatment comes from your GP or a specialist instead.

• Eat enough for your activity. Under-eating and very low body weight push SHBG up and testosterone down. If you train hard, fuel it with regular meals that include protein and carbohydrate.
• Lose excess weight around the waist if you carry it. In men this tends to lift total and bioavailable testosterone, and regular strength training adds to the gain in insulin sensitivity.
• Drink less. Heavy drinking lowers testosterone and, through its effect on the liver (where SHBG is made), can push SHBG up.
• Get heavy snoring checked. Untreated sleep apnoea lowers testosterone.
• Women with PCOS: regular movement and meals built around protein, fibre and slow carbohydrates can improve insulin resistance, which raises SHBG and lowers available androgens.

When to see your GP

See your GP if two early morning tests both come back below your lab's range, especially with low libido, erectile problems, fewer morning erections, low mood or loss of muscle.

Book in promptly if you have a low result plus headaches or changes in your vision, which can signal a pituitary problem. Women should also go promptly for fast-spreading facial or body hair, a deeper voice, or periods that have stopped for three months when not pregnant.

Testing bioavailable testosterone with Vively

Bioavailable testosterone is included in our Male Hormone Health Test, Female Hormone Health Test, Perimenopause and Menopause Hormone Tests, Weight Management Test and Full Body Health Check, alongside SHBG and total testosterone. Start with your Baseline Health Check, then add whichever test fits. The exact markers can depend on the referring doctor's review.

After we arrange the referral through our GP network, your sample is taken at one of 3,700+ accredited partner collection centres: book early morning if you're a man, or day 2 to 5 of your cycle if you have periods. Results land in the app, usually within a few days, against optimal ranges. Your practitioner explains what your SHBG pattern means, builds a personalised plan with you and sets a retest.

Men can order the Male Hormone Health Test, and women the Female Hormone Health Test.

Frequently asked questions

Do I need to fast before a bioavailable testosterone test?

No fasting is needed for the marker itself, but men should test in the morning, ideally before breakfast and before 10am. Women with periods should test on day 2 to 5. Vively's Baseline needs an overnight fast for glucose and insulin, so if you test with it, you'll be fasting anyway.

Is bioavailable testosterone the same as free testosterone?

No. Free testosterone counts only unbound testosterone. Bioavailable testosterone is free testosterone plus the share loosely bound to albumin, so it is a larger number and is reported in nmol/L instead of pmol/L. Both are calculated from total testosterone, SHBG and albumin, and they usually rise and fall together.

Is a bioavailable testosterone of 2.0 nmol/L low?

For a man, it is below the typical lower limit of about 2.5 nmol/L. Repeat the test early in the morning before reading much into it, and look at SHBG, total testosterone, LH and symptoms. For a woman, 2.0 nmol/L would be above the usual range and worth reviewing.

Why is my total testosterone low but my bioavailable testosterone normal?

This usually means your SHBG is low, so less testosterone is locked away and more is available. It is common in men carrying extra weight or with insulin resistance. Your usable testosterone may be fine, but the low SHBG is worth acting on for your metabolic health.

Can the pill affect bioavailable testosterone?

Yes. The combined oral contraceptive pill raises SHBG, which lowers bioavailable and free testosterone. Your result reflects the pill rather than what your ovaries would produce without it. Oral HRT has a similar effect. Tell your practitioner what contraception or hormones you use.

Can I get a bioavailable testosterone test through Medicare?

Many blood tests can be Medicare-funded when a doctor orders them for a clinical reason. With Vively, we arrange the referral through our GP network as part of your hormone test, so you don't need to book a separate GP appointment first.

References

• Royal College of Pathologists of Australasia (RCPA). RCPA Manual: Testosterone.
• Royal College of Pathologists of Australasia (RCPA). RCPA Manual: Sex hormone binding globulin (SHBG).
• Pathology Tests Explained. Sex hormone binding globulin (SHBG). https://www.pathologytestsexplained.org.au
• Healthy Male. Androgen deficiency: clinical summary guide. https://www.healthymale.org.au
• 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.
• Yeap BB, Grossmann M, McLachlan RI, et al. Endocrine Society of Australia position statement on male hypogonadism (part 1): assessment and indications for testosterone therapy. Medical Journal of Australia. 2016;205(4):173-178.
• Teede HJ, Tay CT, Laven JJE, et al. Recommendations from the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome. Journal of Clinical Endocrinology and Metabolism. 2023.