Free Testosterone
For informational purposes only — not medical advice. Always consult a qualified healthcare provider before making changes to your health regimen. Full disclaimer →
- Free testosterone is what actually matters biologically — total testosterone tells you what's in the blood; free testosterone tells you what's available to cells. The distinction is clinically significant.
- SHBG rises with age, illness, and certain lifestyle factors — directly reducing free testosterone even when total testosterone remains stable. A man's free testosterone can drop 50% from age 40 to 65 while total testosterone only drops 20%.
- Always test total testosterone, free testosterone, and SHBG together. Without SHBG, you can't accurately calculate free testosterone. Without all three, the picture is incomplete.
- Symptoms of low testosterone with normal total testosterone warrant a free testosterone measurement. This is the most common scenario in which a symptomatic man is told his levels are 'fine' — because only total testosterone was measured.
- Several factors raise SHBG and therefore lower free testosterone: aging, low thyroid function, elevated estradiol, high fiber intake (to a minor extent), liver disease, and caloric restriction.
Why Total Testosterone Is Only Half the Story
When a man complains of fatigue, low libido, difficulty building muscle, mood changes, or cognitive fog — the classic symptoms of testosterone deficiency — the standard medical workup orders total testosterone. If it comes back within the reference range, the conversation often ends: "Your testosterone is normal."
This represents a fundamental gap in how androgen status is evaluated. Total testosterone measures all testosterone in the blood — including the 60–70% that is tightly bound to SHBG (sex hormone-binding globulin) and biologically unavailable to cells. The relevant fraction — the testosterone that can actually enter cells, bind to androgen receptors, and drive biological effects — is free testosterone, which makes up only 1–3% of the total.
A man with total testosterone of 600 ng/dL and SHBG of 80 nmol/L has a calculated free testosterone of approximately 8–9 pg/mL — in the lower quartile of normal and likely functionally insufficient for a 45-year-old man trying to maintain muscle mass, libido, and cognitive function. A man with total testosterone of 500 ng/dL and SHBG of 25 nmol/L has free testosterone of approximately 16–18 pg/mL — substantially higher despite lower total testosterone. These two men will have dramatically different physiological experiences from their androgen levels, yet a total testosterone test alone cannot distinguish them.
A study in the Journal of Clinical Endocrinology & Metabolism found that free testosterone was a better predictor of hypogonadal symptoms than total testosterone in older men, and that a significant proportion of symptomatic men had normal total testosterone but low free testosterone — a pattern missed by total testosterone screening alone. 1
How Free Testosterone Changes With Age
Total testosterone declines approximately 1–2% per year after age 30 in men. Free testosterone declines significantly faster — approximately 2–3% per year — because SHBG rises with age, binding an increasing proportion of total testosterone and leaving less free.
The practical consequence is that a man who might tolerate a 20% decline in total testosterone without significant symptoms could simultaneously experience a 40–50% decline in free testosterone and be functionally androgen-deficient while appearing normal on standard testing.
| Age (Men) | Standard Range (pg/mL) | Longevity Target |
|---|---|---|
| 25–34 | 15.0–40.0 | Upper third: > 27 |
| 35–44 | 12.0–30.0 | Upper third: > 22 |
| 45–54 | 9.0–26.0 | Upper third: > 18 |
| 55–64 | 8.0–21.0 | Upper third: > 15 |
| 65+ | 6.0–18.0 | Upper third: > 12 |
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Analyze My Biomarkers →The SHBG Problem — and What Raises It
Understanding what drives SHBG is essential for anyone interpreting testosterone results. SHBG is produced by the liver and regulated by multiple hormonal and metabolic inputs.
What raises SHBG (lowering free testosterone):
- Aging — SHBG rises approximately 1–2% per year throughout adulthood; this is the dominant driver of declining free testosterone in older men
- Thyroid hormone excess (hyperthyroidism) — thyroid hormones strongly stimulate SHBG production; elevated free T4 or T3 can substantially raise SHBG
- Estrogen — endogenous elevated estradiol and exogenous estrogen (including oral contraceptives in women) raise SHBG significantly
- Liver disease — cirrhotic and inflamed liver tissue produces elevated SHBG
- Low insulin states — fasting and very low caloric intake transiently reduce insulin, which raises SHBG
What lowers SHBG (raising free testosterone):
- Insulin resistance and obesity — high insulin directly suppresses hepatic SHBG production
- Hypothyroidism — low thyroid hormone reduces SHBG
- Anabolic steroid use — exogenous androgens suppress SHBG
- Glucocorticoids — cortisol and corticosteroids suppress SHBG
- Growth hormone — IGF-1 reduces SHBG
This complexity means that interpreting testosterone without SHBG is incomplete — two men with identical total testosterone can have free testosterone that differs by a factor of two or more based on their SHBG levels.
What to Do With a Low Free Testosterone
If free testosterone is low — particularly in the context of symptoms — the clinical approach depends on the underlying mechanism:
If SHBG is elevated with normal total testosterone: The priority is identifying and treating the cause of high SHBG. Check thyroid function (free T4 and T3), assess for liver disease, review medications, and evaluate whether estradiol is elevated. Resistance training and maintaining healthy insulin sensitivity help modulate SHBG over time.
If both total testosterone and free testosterone are low: This points toward primary or secondary hypogonadism — either the testes aren't producing enough testosterone or the HPG axis isn't driving adequate production. Evaluation by a physician experienced in male hormonal health (often an endocrinologist or urologist specializing in men's health) is appropriate. Testosterone replacement therapy is the most direct intervention and is highly effective for appropriately selected patients.
If free testosterone is borderline and symptoms are present: A full hormonal panel — including LH, FSH, prolactin, estradiol, DHEA-S, and thyroid function — provides the most complete picture before considering any intervention. Marek Health and similar longevity-focused telehealth providers offer comprehensive hormonal evaluation with clinician interpretation.
Sources
- Vermeulen A, et al. "A Critical Evaluation of Simple Methods for the Estimation of Free Testosterone in Serum." Journal of Clinical Endocrinology & Metabolism, 1999. PubMed →
- Harman SM, et al. "Longitudinal Effects of Aging on Serum Total and Free Testosterone Levels in Healthy Men." Journal of Clinical Endocrinology & Metabolism, 2001. PubMed →
- Morley JE, et al. "Validation of a Screening Questionnaire for Androgen Deficiency in Aging Males." Metabolism, 2000. PubMed →
| Range Type | Value (pg/mL) | Notes |
|---|---|---|
| Standard Clinical Range | 9.3–26.5 pg/mL (age-dependent) | Designed to identify disease risk — not longevity optimisation. |
| Longevity-Optimal Target | Upper third of age-adjusted range: > 15 pg/mL (men 40–60) |
Associated with reduced all-cause mortality and extended healthspan.
Free testosterone must be interpreted against age-adjusted norms. The calculated free testosterone method (using total testosterone, SHBG, and albumin) is generally preferred over direct immunoassay, which is less accurate at lower levels. A combined assessment of total testosterone, free testosterone, and SHBG provides the most complete picture of androgen status.
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How is free testosterone calculated vs. directly measured?
Free testosterone can be measured directly by equilibrium dialysis (the gold standard method) or calculated using a validated formula that takes total testosterone, SHBG, and albumin as inputs. Direct immunoassay methods for free testosterone are widely available but notoriously inaccurate at lower values, which is exactly where clinical decisions need to be made. The calculated free testosterone using the Vermeulen equation or the SHBG-based calculation is generally more reliable than direct immunoassay for clinical purposes. When ordering free testosterone, ask whether the lab uses equilibrium dialysis (most accurate, often sent to specialized labs) or direct immunoassay (less reliable). If the test returns a 'calculated free testosterone' value based on total T and SHBG, this is often more meaningful than a direct measurement by immunoassay.
What is SHBG and what affects it?
SHBG (sex hormone-binding globulin) is a protein produced by the liver that binds tightly to testosterone (and estradiol), carrying it through the bloodstream but rendering it biologically unavailable to cells. SHBG levels are affected by many factors: they rise with aging (contributing to declining free testosterone in older men), thyroid hormone excess (hyperthyroidism significantly elevates SHBG), estrogen (endogenous or exogenous), liver disease, and chronic illness. SHBG falls with insulin resistance and obesity (one reason obese men often have low total testosterone), anabolic steroid use, high insulin levels, hypothyroidism, and nephrotic syndrome. Because SHBG mediates the relationship between total and free testosterone, it is an essential component of any complete androgen assessment.
What free testosterone level is considered low?
Reference ranges for free testosterone vary by lab and measurement method. Generally, free testosterone below 9 pg/mL (or the lower quartile for age) is considered low, and above 15 pg/mL is generally adequate for most men aged 40–60. However, because free testosterone affects each person differently depending on androgen receptor sensitivity, symptoms in context matter as much as absolute values. A man with free testosterone of 10 pg/mL who is asymptomatic may not need intervention; a man with the same value experiencing significant fatigue, low libido, and loss of muscle mass probably does. The combination of low-normal free testosterone plus symptoms is the typical clinical scenario that warrants discussion of treatment.
How can I naturally increase free testosterone?
Strategies to raise free testosterone work through two mechanisms: raising total testosterone production or lowering SHBG. For total testosterone: resistance training (particularly compound lifts — squats, deadlifts, bench press) is the most potent natural stimulus for testosterone production; adequate sleep (testosterone is primarily produced during REM sleep); maintaining healthy body weight (obesity reduces testosterone); reducing chronic stress (cortisol suppresses the HPG axis); adequate dietary fat and zinc. For lowering SHBG specifically: improving insulin sensitivity through exercise and low-carbohydrate diet (high insulin reduces SHBG); optimizing thyroid function; maintaining healthy body weight; adequate protein intake. Stinging nettle root has some evidence for modestly reducing SHBG, though the clinical magnitude is debated.
Should women test free testosterone?
Yes — free testosterone is relevant for women as well as men. Women produce testosterone primarily in the ovaries and adrenal glands, and it plays important roles in libido, energy, mood, muscle maintenance, and bone density. Female testosterone levels are approximately 10–15% of male levels, but free testosterone is the relevant fraction. Low free testosterone in women — which can occur due to elevated SHBG from oral contraceptives (OCP use dramatically raises SHBG), aging, or adrenal insufficiency — is associated with reduced libido, fatigue, and impaired muscle function. Women on oral contraceptives should be aware that OCPs raise SHBG significantly, which can substantially reduce free testosterone even if total testosterone is normal.