Last updated: June 2026 · Reviewed June 2026 · Built by the InjectBuddy team
What do SHBG and albumin do? How they control your testosterone
SHBG and albumin are the two blood proteins that bind testosterone, and together they decide how much is free to act on your tissues. SHBG binds tightly and locks testosterone away, albumin binds loosely and lets it go easily — so the balance between them sets your free and bioavailable testosterone, and both feed the calculated free-T formula. This guide explains how each protein works, compares their binding behavior side by side, walks through worked examples showing how SHBG and albumin move your numbers, and answers the questions people ask most.
- Only about 1–2% of testosterone circulates truly free; the rest is bound to SHBG (tightly) and albumin (loosely).
- High SHBG → lower free T. Tightly bound testosterone is not available to tissues.
- Bioavailable T = free + albumin-bound, because albumin releases testosterone easily.
- The Vermeulen free-T equation needs total T, SHBG, and albumin as inputs.
- Run your own numbers in the free testosterone index calculator.
The two binding proteins, side by side
Picture testosterone in the bloodstream as commuters. SHBG is a small number of locked taxis that hold a passenger firmly and rarely let go. Albumin is a huge fleet of open buses that pick passengers up but drop them at every stop. A tiny fraction of commuters walk freely with no transport at all — that walking fraction is your free testosterone, and it is the part that actually reaches androgen receptors.
The numbers behind the analogy matter. Sex hormone-binding globulin (SHBG) binds testosterone with roughly a thousand-fold higher affinity than albumin, but there is far more albumin in blood, so albumin still carries a large share of the total (Vermeulen et al., 1999). Because albumin’s grip is weak, the testosterone it carries dissociates at the capillary and is treated as usable. SHBG-bound testosterone is effectively sidelined. This is why two men with identical total testosterone can have very different symptoms: the one with high SHBG has less free hormone doing work.
Free vs bioavailable testosterone
Free testosterone is the unbound fraction only. Bioavailable testosterone is the free fraction plus the albumin-bound fraction, since albumin readily surrenders its cargo. That single distinction explains most confusion in Testosterone (TRT) bloodwork: bioavailable T is always a larger number than free T, and which one your lab reports changes the reference range you compare against. Guidelines recommend estimating free T by a validated formula or equilibrium dialysis whenever total T is borderline or SHBG is abnormal (Bhasin et al., 2018).
SHBG vs albumin at a glance
| Property | SHBG | Albumin |
|---|---|---|
| Binding affinity | High (tight) | Low (loose) |
| Typical % of total T bound | ~44–65% | ~33–54% |
| Counts as bioavailable? | No — locked away | Yes — released easily |
| Effect of a rise on free T | Lowers free T | Minimal at normal levels |
| Usual blood level | ~10–57 nmol/L | ~35–50 g/L |
| Needed by the free-T formula? | Yes | Yes |
The percentages overlap because they depend on each person’s SHBG level: the higher the SHBG, the bigger its slice and the smaller the free and albumin-bound slices. Albumin is fairly stable, so day-to-day swings in free T are mostly driven by SHBG and total testosterone.
How this is calculated
Calculated free testosterone treats the blood as three pools in chemical equilibrium: free, SHBG-bound, and albumin-bound. The Vermeulen formula solves for the free fraction using total testosterone, SHBG, and albumin, applying the known binding constants for each protein (Vermeulen et al., 1999). You do not need to do the algebra by hand — the free testosterone index calculator runs it — but understanding the inputs stops you from misreading the output.
A simpler cousin is the Free Androgen Index (FAI), which is just total testosterone ÷ SHBG × 100. It ignores albumin entirely, which is why it tracks the Vermeulen free T loosely but not exactly. The worked examples below show how each input moves the answer.
Total testosterone 20 nmol/L. If roughly 2% is truly free, free T ≈ 20 × 0.02 = 0.4 nmol/L (400 pmol/L). The other ~98% is bound to SHBG and albumin.
Two men both have total T 20 nmol/L. Man A: SHBG 25 nmol/L. Man B: SHBG 60 nmol/L. Using FAI = T ÷ SHBG × 100: A = 20÷25×100 = 80; B = 20÷60×100 = 33. Same total, less than half the free index.
Total T 18 nmol/L, SHBG 40 nmol/L. FAI = 18 ÷ 40 × 100 = 45. A normal male FAI is roughly 35–100, so this sits in range despite a mid-level total.
Low albumin (e.g. 30 g/L vs a default 43 g/L) means less loosely bound testosterone, so the calculated free fraction rises slightly. Entering your real albumin instead of the 43 g/L default can shift calculated free T by a few percent — small, but real.
If free T is 0.4 nmol/L and the albumin-bound pool is about 9 nmol/L on a 20 nmol/L total, bioavailable T ≈ 0.4 + 9 = 9.4 nmol/L — over 20× the free number. Never compare a bioavailable result to a free-T range.
Labs report total T in either unit. To convert ng/dL to nmol/L, divide by 28.84: 600 ng/dL ÷ 28.84 = 20.8 nmol/L. Feed the calculator matching units or the free-T output is meaningless.
SHBG climbs with age, so an older man with total T 16 nmol/L and SHBG 55 may have a lower free T than a younger man at total T 14 nmol/L and SHBG 30: FAI 29 vs 47. Free hormone, not total, often explains symptoms.
At SHBG 20 nmol/L the free fraction might be ~2.5%; at SHBG 80 nmol/L it can fall below 1%. So on total T 20 nmol/L, free T moves from ~0.50 to ~0.18 nmol/L purely from SHBG — albumin and total unchanged.
What moves SHBG (and therefore free T)
Albumin is fairly steady unless you are unwell, so most free-T variation comes from SHBG. SHBG rises with age, oral estrogen, hyperthyroidism, liver disease and low body weight. It falls with obesity, insulin resistance, hypothyroidism and high androgen levels. Because SHBG sets how much testosterone is locked away, any of these can change your free T even when total T is steady — which is exactly why guidelines ask for a free-T estimate when SHBG is abnormal (Bhasin et al., 2018). Most testosterone circulates protein-bound, with only a small free fraction biologically active at any moment (Physiology, Testosterone, StatPearls).
This is a maths reference, not medical advice. A calculated free T is only as good as the lab values you enter, and it does not diagnose anything on its own. If you are drawing blood at home, follow safe injection and sample-handling practice (CDC).
So, what do SHBG and albumin do to testosterone?
SHBG and albumin are the two blood proteins that bind most of your circulating testosterone, leaving only about 1–2% truly free. SHBG binds tightly and sequesters testosterone so it cannot reach androgen receptors; albumin binds loosely and releases its cargo at the tissue, so albumin-bound testosterone counts as bioavailable. The balance between them is captured by the Vermeulen equation — free T = f(total T, SHBG, albumin) — and a shift in SHBG alone can halve your free testosterone even when total T is unchanged. Enter your own lab values into the free testosterone index calculator to see exactly how SHBG and albumin affect your numbers.
FAQs
What do SHBG and albumin do to testosterone?
Do SHBG and albumin both lower free testosterone?
Why does the free testosterone formula need albumin?
Is bioavailable testosterone the same as free testosterone?
What raises or lowers SHBG?
Sources
- 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. PubMed PMID: 10523012.
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab. 2018. PubMed PMID: 29562364.
- Nassar GN, Leslie SW. Physiology, Testosterone. StatPearls. NCBI Bookshelf NBK526128.
- CDC. Safe Injection Practices to Prevent Transmission of Infections to Patients. CDC injection safety guidance.
This guide is for general educational purposes only and does not constitute medical advice. SHBG and albumin levels, and any calculated free testosterone, must be interpreted by your prescriber alongside your symptoms and reference ranges.