Bioavailable Testosterone Calculator

Total testosterone measurement captures all forms of the hormone in blood — tightly bound to SHBG (which cannot enter target cells), loosely bound to albumin (which can dissociate and enter cells), and the tiny free fraction. When a middle-aged man with symptoms of hypogonadism (fatigue, reduced libido, decreased muscle mass) has "normal" total testosterone, the bioavailable fraction — free plus albumin-bound — is often the missing diagnostic piece. The bioavailable testosterone calculator uses the Vermeulen equilibrium equations to compute these clinically relevant fractions.

The Three Testosterone Fractions

Testosterone in blood exists in three bound states:

• SHBG-bound (approximately 40–60% of total T): tightly bound to sex hormone-binding globulin with dissociation constant Kd ≈ 5.97 × 10⁻⁹ mol/L; this fraction cannot enter target tissues and is biologically inactive
• Albumin-bound (approximately 38–54% of total T): weakly bound to albumin (Kd ≈ 3.6 × 10⁻⁴ mol/L); this binding is easily reversible and this fraction is available to target tissues
• Free testosterone (approximately 1–3% of total T): unbound; directly available for cell entry; the most biologically potent fraction per unit

Bioavailable testosterone = free T + albumin-bound T — the clinically relevant measure of androgen availability. Use this online calculator with your lab values to compute both free and bioavailable fractions. The free testosterone calculator focuses specifically on the free fraction.

The Vermeulen Equations: The Calculation Standard

The Vermeulen et al. (1999) method — recommended by the Endocrine Society — uses equilibrium binding equations:

The fraction of testosterone unbound (f_free) satisfies the equilibrium equation accounting for both SHBG and albumin binding simultaneously. The final calculation:

• Free T (nmol/L) = Total T × f_free
• Albumin-bound T = Total T × (albumin × Ka) × f_free / (1 + albumin × Ka + SHBG × Kd)
• Bioavailable T = Free T + Albumin-bound T

where standard albumin = 4.3 g/dL = 43 g/L = 0.643 mmol/L; Kd(SHBG-T) = 5.97 × 10⁻⁹ mol/L; Ka(albumin-T) = 3.6 × 10⁴ L/mol = 1/Kd. Reference ranges: free T in adult men: 5–21 pg/mL (174–729 pmol/L); bioavailable T: 100–350 ng/dL (3.47–12.14 nmol/L). Note: this calculator is for educational understanding — always interpret results in clinical context with a qualified healthcare provider.

SHBG and Its Clinical Significance

SHBG is produced by the liver and its concentration is the primary determinant of bioavailable testosterone variability between individuals with similar total T. Conditions that raise SHBG (reducing bioavailable T):

• Aging (SHBG increases approximately 1–2% per year after age 40)
• Hepatic disease, cirrhosis
• Hyperthyroidism
• Estrogen exposure (oral contraceptive pills, gender-affirming therapy)

Conditions that lower SHBG (increasing bioavailable T fraction):

• Obesity and insulin resistance (insulin suppresses SHBG production)
• Type 2 diabetes
• Hypothyroidism
• Anabolic steroid use

The SHBG calculator and hormone calculators provide complementary endocrinology tools. This content is for educational purposes only — testosterone testing and interpretation requires clinical evaluation by a qualified physician.

Clinical Decision-Making: When to Measure Bioavailable T

The Endocrine Society Clinical Practice Guidelines (2018) recommend measuring free or bioavailable testosterone when total testosterone is in the borderline range (200–400 ng/dL) or when SHBG abnormality is suspected. Conditions warranting bioavailable T testing: obese men (low SHBG may make total T appear falsely low relative to bioavailable fraction); elderly men (high SHBG may make total T appear adequate while bioavailable T is deficient); men with chronic illness affecting SHBG; and men with symptoms disproportionate to their total T level. A total T in the normal range (above 350 ng/dL) combined with low bioavailable T suggests SHBG excess, not hypogonadism — replacement therapy is not indicated in this scenario without further evaluation.