Free testosterone calculator: total T, SHBG, and albumin

Most lab reports give total testosterone, and many add SHBG. The free testosterone figure, the small share not bound to any protein, often has to be calculated from those two numbers and albumin. Enter your results below to get free and bioavailable testosterone by the Vermeulen equation, the calculation most often used when free testosterone isn’t measured directly.

What to enter

• Total testosterone. In ng/dL (common in the US) or nmol/L. Use an early-morning, fasting result if you have one, because testosterone falls through the day and after a meal.
• SHBG (sex hormone-binding globulin), in nmol/L. It’s usually reported in nmol/L everywhere. Use the SHBG from the same blood draw as the total testosterone.
• Albumin. In g/dL (US) or g/L. It’s part of most metabolic panels. If you don’t have it, leave the default of 4.3 g/dL, a typical adult value and the same default the ISSAM calculator uses. Albumin moves the result much less than SHBG does.

All three need to come from the same draw. SHBG and albumin both change over time, so pairing this year’s testosterone with last year’s SHBG gives a number that doesn’t describe either day.

Free and bioavailable testosterone

Testosterone travels in the blood in three forms:

• Bound to SHBG. Held tightly, and not available to tissues. Usually the largest share.
• Bound to albumin. Held loosely, and released easily as blood passes through tissues.
• Free. Not bound at all. Usually only a few percent of the total, often around 2.

Free testosterone is the unbound part only. Bioavailable testosterone is the free part plus the part loosely bound to albumin, on the view that both can reach tissues. Both are reported in smaller units than total testosterone, usually pg/mL or pmol/L for free and ng/dL or nmol/L for bioavailable.

The free number matters most when SHBG is unusual. Obesity, type 2 diabetes, and some medicines lower SHBG, which lowers total testosterone while the free part may stay normal. Older age, liver disease, an overactive thyroid, and some anticonvulsants raise SHBG, which can hide a low free testosterone behind a normal total. The Endocrine Society guideline recommends checking free testosterone in those situations, and in men whose total testosterone is near the lower limit.

How the calculation works

SHBG and albumin each bind testosterone with a known strength. Given how much testosterone, SHBG, and albumin are in the blood, the law of mass action fixes how much of the testosterone must be left unbound. The calculator solves that equation, as Vermeulen, Verdonck, and Kaufman described in 1999, with their binding constants: 1.0 × 10⁹ L/mol for SHBG and 3.6 × 10⁴ L/mol for albumin. With every concentration in mol/L (albumin in g/L divided by 69,000):

N  = 1 + 3.6 × 10⁴ × albumin
b  = N + 10⁹ × (SHBG − total)
free = (−b + √(b² + 4 × 10⁹ × N × total)) ÷ (2 × 10⁹ × N)
bioavailable = N × free

Bioavailable testosterone is the free result plus the albumin-bound share that follows from it, which is what N × free adds up to.

As a check, a total testosterone of 288 ng/dL (10 nmol/L) with an SHBG of 40 nmol/L and an albumin of 4.3 g/dL gives a free testosterone of about 50 pg/mL, which is 1.74 percent of the total, and a bioavailable testosterone of about 118 ng/dL.

Calculated or measured

There are three ways a lab can give you a free testosterone number:

• Equilibrium dialysis measures it directly. It’s the reference method, but it’s slow and costly, so it’s usually a send-out test.
• Calculated from total testosterone, SHBG, and albumin, as here. Vermeulen and colleagues found the calculation tracked equilibrium dialysis closely, which is why it’s widely used.
• “Direct” or “analog” immunoassay. A one-step kit. The Endocrine Society guideline recommends against it because it is inaccurate.

If your report says “free testosterone” without saying how, check the test name or the lab’s test directory. Words like “dialysis”, “calculated”, or “direct” in the name tell you which one you have. Results from different methods shouldn’t share a trend line.

Limits of the calculation

• It’s only as good as its inputs. The calculated value depends heavily on SHBG, and SHBG assays differ between labs. A change of lab can move the calculated free testosterone even when nothing about you changed.
• Other equations exist. Some labs use different binding constants or a different model of how SHBG binds testosterone, and they give somewhat different numbers from the same inputs. Compare results calculated the same way.
• Low levels need an accurate total. Women’s testosterone, and men’s on some treatments, is low enough that many immunoassays for total testosterone are inaccurate. A calculated free value inherits that error. For women, the Endocrine Society position statement on testosterone testing recommends mass spectrometry for the total.

Tracking free testosterone over time

A single free testosterone number has the same problems as a single total: the time of day, food, sleep, illness, and the lab all move it. Calculate each result from the total, SHBG, and albumin of the same draw, keep the method the same, and chart free testosterone on its own line, separate from total. How to track your testosterone over time covers draw timing, the reference ranges, and what counts as a real change.

Tracking testosterone in HealthViewer

HealthViewer is a lab results tracker that runs on your own computer. It imports testosterone, SHBG, and albumin results from lab report PDFs, MyChart downloads, and direct connections to your health system. Total testosterone and free testosterone are kept as separate tests, and total testosterone in nmol/L is converted so it shares one chart with results in ng/dL.

In Trends, the overlay view puts total testosterone, free testosterone, and SHBG on one chart, so you can see whether a drop in total came with a drop in SHBG.

Everything stays in an encrypted file on your computer, with no account and no cloud service. Getting Started walks through the first import.

This calculator is for reading results, not for diagnosis or treatment decisions. A calculated free testosterone is an estimate. Talk to your clinician about your results and before starting, stopping, or changing any treatment.