Two Numbers One Hormone
When your doctor orders a testosterone test, you typically get one number: total testosterone. This measures all the testosterone in your blood, including the 98% that is bound to proteins and unavailable for your body to use. Only about 2-3% of your testosterone is free, meaning unbound and biologically active. This free fraction is what actually enters cells, binds to androgen receptors, and produces the effects you associate with testosterone.
Total testosterone is like knowing your gross salary. Free testosterone is your take-home pay. One looks good on paper. The other is what you actually live on.
What SHBG Does
Sex hormone-binding globulin is a protein produced by your liver that binds testosterone tightly, making it unavailable for biological activity. Approximately 65-80% of your total testosterone is bound to SHBG. Another 20-30% is loosely bound to albumin and can become free. Only 2-3% is truly free at any given time.
SHBG levels vary significantly between individuals and are affected by age, body weight, liver function, thyroid status, and certain medications. This variability is why two men with identical total testosterone can feel dramatically different. The man with low SHBG has more free testosterone available. The man with high SHBG has less, despite the same total number.
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The most common scenario where total testosterone misleads is in men with elevated SHBG. This occurs frequently with aging, liver conditions, hyperthyroidism, and use of certain medications including some antiepileptics. A man with total testosterone of 500 ng/dL and SHBG of 60 nmol/L may have a free testosterone level comparable to a man with total testosterone of 300 and normal SHBG. Both are symptomatic, but only the second one looks low on a standard test.
Conversely, men with very low SHBG, common in obesity and insulin resistance, may have adequate free testosterone despite a low total testosterone. This is why testing both numbers is essential for an accurate picture.
Optimal Free T Ranges
Optimal free testosterone for adult men is generally 9-25 pg/mL, though some labs report in different units. Below 9 pg/mL, most men experience symptoms regardless of total testosterone. The sweet spot for most men is 15-25 pg/mL, where energy, libido, body composition, and cognition are at their best.
Getting the Right Test
Always request both total and free testosterone on any hormone panel. Calculated free testosterone derived from total testosterone and SHBG is acceptable, but directly measured free testosterone by equilibrium dialysis is more accurate. Along with these, SHBG and albumin levels provide the complete picture needed to understand your hormonal status. Never make treatment decisions based on total testosterone alone.
This article is informed by peer-reviewed research and clinical guidelines:
- Lincoff AM, Bhasin S, Flevaris P, et al. Cardiovascular Safety of Testosterone-Replacement Therapy. N Engl J Med 2023;389:107-117. View study →
- Mulhall JP, Trost LW, Brannigan RE, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. J Urol 2018;200:423-432. View guideline →
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2018;103:1715-1744. View guideline →
- Snyder PJ, Bhasin S, Cunningham GR, et al. Effects of Testosterone Treatment in Older Men (Testosterone Trials). N Engl J Med 2016;374:611-624. View study →
All Heyday Health content is reviewed by licensed providers and updated when clinical guidelines change. See our medical team for review credentials.