Testosterone Test: Total, Free, and Bioavailable

Testosterone is the principal androgenic hormone in both sexes, essential for reproductive function, muscle mass, bone density, mood regulation, cognitive function, and metabolic health. Accurate assessment requires measuring not just total testosterone but also free and bioavailable fractions, as well as the binding protein SHBG, to determine how much testosterone is actually available to tissues.

Table of Contents

1. Overview
2. When Ordered
3. Reference Ranges
4. Low Testosterone Symptoms in Men
5. High Testosterone in Women and PCOS
6. Age-Related Decline
7. Testosterone Replacement Therapy (TRT) Considerations
8. Natural Optimization Strategies
9. References

Overview

Testosterone is a steroid hormone synthesized primarily in the Leydig cells of the testes in men and, to a lesser extent, in the ovaries and adrenal glands in women. Hypothalamic GnRH stimulates pituitary LH release, which in turn drives testicular testosterone production — a classic feedback loop regulated by circulating testosterone levels. In women, testosterone is produced in smaller amounts by the ovaries and adrenal cortex, where it serves as a precursor to estradiol and exerts direct androgenic effects.

Once released into circulation, testosterone circulates in three fractions. Approximately 44–68% is tightly bound to sex hormone-binding globulin (SHBG) and is biologically inactive. Another 30–54% is loosely bound to albumin and is considered bioavailable, as it can dissociate at the tissue level. Only 1–4% circulates as free (unbound) testosterone, which can directly enter cells and bind androgen receptors. The clinically relevant distinction between total testosterone and free/bioavailable testosterone becomes critical when SHBG levels are abnormal — elevated SHBG (as seen with aging, hyperthyroidism, or liver disease) can leave a patient symptomatic despite a normal total testosterone level.

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When Ordered

Testosterone testing is indicated in a wide range of clinical presentations:

• Evaluation of hypogonadism symptoms in men: fatigue, decreased libido, erectile dysfunction, decreased muscle mass, increased body fat, mood changes, or poor concentration
• Evaluation of androgen excess in women: acne, hirsutism, menstrual irregularities, clitoromegaly, or suspected polycystic ovary syndrome (PCOS)
• Assessment of infertility in both sexes
• Evaluation of delayed or precocious puberty in adolescents
• Monitoring of testosterone replacement therapy (TRT) in men
• Evaluation of decreased bone mineral density or unexplained osteoporosis in men
• Assessment of pituitary or hypothalamic dysfunction
• Follow-up of abnormal LH or FSH levels
• Monitoring anti-androgen therapy in prostate cancer treatment

Testosterone levels follow a diurnal rhythm, peaking in the early morning (7–10 AM) and declining by 20–30% through the afternoon. Blood samples should be drawn before 10 AM for the most accurate and reproducible results, particularly for borderline cases.

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Reference Ranges

Total Testosterone — Male (ng/dL)

Total Testosterone — Female (ng/dL)

Free Testosterone — Male (pg/mL)

SHBG — Male (nmol/L)

Reference ranges are age-dependent. Men aged 20–30 typically have total testosterone of 600–1000 ng/dL, while men aged 60–80 may have levels of 300–500 ng/dL. The threshold for clinically significant hypogonadism is generally set at total testosterone below 300 ng/dL, though symptoms at levels between 300–400 ng/dL in conjunction with low free testosterone should also prompt evaluation and potential treatment.

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Low Testosterone Symptoms in Men

Male hypogonadism — clinically low testosterone — produces a constellation of symptoms across multiple organ systems. The condition may be primary (testicular failure with high LH/FSH) or secondary (hypothalamic-pituitary dysfunction with low or normal LH/FSH):

• Sexual function: Decreased libido is typically the earliest symptom. Erectile dysfunction, reduced orgasm intensity, and decreased ejaculate volume reflect androgen dependence of the male sexual response. Low testosterone is a contributing factor in approximately 20% of men with erectile dysfunction.
• Body composition: Testosterone is an anabolic hormone promoting protein synthesis and muscle growth. Low testosterone causes loss of lean muscle mass, increased visceral adiposity, and a shift toward a more estrogenic body habitus. This creates a vicious cycle, as adipose tissue aromatizes testosterone to estradiol, further lowering free testosterone.
• Bone health: Testosterone, through conversion to estradiol, is critical for male bone mineral density. Long-standing hypogonadism leads to osteopenia and osteoporosis, substantially increasing fracture risk.
• Mood and cognition: Testosterone receptors are abundant in limbic system structures. Low testosterone is associated with depressed mood, irritability, poor motivation, reduced assertiveness, and brain fog. It is a frequently overlooked contributor to treatment-resistant depression in middle-aged and older men.
• Energy and sleep: Fatigue, reduced stamina, and poor sleep quality — including reduced REM sleep — are common in hypogonadal men.
• Metabolic effects: Low testosterone is independently associated with insulin resistance, metabolic syndrome, and elevated cardiovascular risk. The relationship is bidirectional — metabolic syndrome suppresses testosterone, and low testosterone worsens metabolic parameters.
• Hematopoiesis: Testosterone stimulates erythropoietin production. Anemia of normocytic type is a recognized consequence of significant hypogonadism in older men.

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High Testosterone in Women and PCOS

Elevated testosterone in women, termed hyperandrogenemia, is the defining hormonal abnormality of polycystic ovary syndrome (PCOS) — the most common endocrine disorder in reproductive-age women, affecting 8–13% of this population. However, hyperandrogenemia in women has multiple causes beyond PCOS:

• Polycystic ovary syndrome: PCOS is characterized by the combination of hyperandrogenism, ovulatory dysfunction, and polycystic ovarian morphology. Elevated testosterone (total or free) is found in approximately 60–80% of PCOS patients. Insulin resistance drives excess LH-stimulated ovarian androgen production. Clinical features include acne, hirsutism (male-pattern hair growth), alopecia, irregular or absent menstrual cycles, and infertility.
• Congenital adrenal hyperplasia (CAH): Non-classical CAH due to 21-hydroxylase deficiency presents in women with hyperandrogenemia and can mimic PCOS. It is distinguished by elevated 17-hydroxyprogesterone levels.
• Adrenal tumors: Androgen-secreting adrenal adenomas or carcinomas produce markedly elevated testosterone and DHEA-S. Rapid onset of virilization should prompt imaging.
• Ovarian hyperthecosis: A condition of excessive ovarian stromal luteinization causing severe hyperandrogenemia, often more pronounced than in PCOS.

Clinical signs of androgen excess in women include acne (particularly cystic or jawline acne), hirsutism (terminal hair on face, chest, abdomen, or inner thighs), androgenic alopecia, clitoral enlargement (in severe cases), and deepening of the voice (virilization). Free testosterone and SHBG measurement is essential in women, as elevated SHBG can mask bioavailable androgen excess even when total testosterone appears normal.

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Age-Related Decline

Testosterone declines progressively with age in both sexes, though the pattern differs significantly between men and women:

• Men: Peak testosterone levels occur in the late teens to mid-twenties. After age 30, total testosterone declines at approximately 1–2% per year — a gradual process termed late-onset hypogonadism or andropause. By age 70, average testosterone levels are roughly 30–40% below peak values. However, SHBG increases with age, meaning free testosterone declines even more steeply — often 2–3% per year — so that men with normal total testosterone may have significantly reduced free testosterone. By age 75, more than 20% of men meet biochemical criteria for hypogonadism.
• Women: Testosterone declines sharply from the mid-twenties onward. By age 40–50, women have approximately half the testosterone of their twenties. Menopause does not cause an abrupt testosterone drop (as it does for estrogen) — the ovaries continue producing some testosterone post-menopause. However, adrenal androgen production (DHEA-S) declines steadily, contributing to progressive androgenic deficiency in postmenopausal women. Low testosterone in women contributes to reduced libido, fatigue, decreased muscle mass, and impaired well-being.

Factors that accelerate testosterone decline beyond normal aging include obesity (aromatase in adipose tissue converts testosterone to estradiol), chronic stress (cortisol suppresses gonadotropin release), sleep deprivation, type 2 diabetes, opioid medications, and glucocorticoid therapy.

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Testosterone Replacement Therapy Considerations

Testosterone replacement therapy (TRT) is an established treatment for confirmed male hypogonadism, though its use in men with borderline levels and symptoms, and in women, remains nuanced. Clinical decision-making requires careful weighing of benefits, risks, and individual patient factors.

Indications and monitoring in men: TRT is most clearly indicated for men with total testosterone below 300 ng/dL on two morning measurements, with consistent symptoms of hypogonadism, after ruling out reversible causes. Available delivery forms include intramuscular injections (testosterone cypionate or enanthate), transdermal gels and patches, subcutaneous pellets, and oral testosterone undecanoate. Monitoring should include testosterone levels (aiming for mid-normal range), hematocrit (testosterone stimulates erythropoiesis; elevated hematocrit increases thrombosis risk), prostate-specific antigen (PSA), and bone density in patients with osteopenia.

Key risks and contraindications:

• Erythrocytosis: Polycythemia is the most common dose-limiting side effect, particularly with injectable forms. Hematocrit above 54% requires dose reduction or treatment interruption.
• Fertility suppression: Exogenous testosterone suppresses FSH and LH via negative feedback, profoundly reducing sperm production. Men desiring fertility preservation should use clomiphene citrate or human chorionic gonadotropin (hCG) instead of, or alongside, TRT.
• Cardiovascular effects: The cardiovascular safety of TRT has been extensively debated. Recent large RCT data (TRAVERSE trial) suggest no increased risk of major adverse cardiovascular events in eugonadal-range TRT, though elevated hematocrit remains a concern for thrombotic events.
• Prostate health: TRT is contraindicated in active prostate cancer. It does not appear to initiate de novo prostate cancer but may stimulate growth of subclinical disease. PSA monitoring is mandatory.
• Aromatization: A portion of exogenous testosterone converts to estradiol via aromatase, potentially causing gynecomastia or fluid retention. Aromatase inhibitors may be added if estradiol rises excessively.

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Natural Optimization Strategies

For individuals with low-normal testosterone or those seeking to maintain healthy levels, several evidence-based lifestyle and nutritional interventions can meaningfully support testosterone production:

• Sleep optimization: The majority of daily testosterone secretion occurs during sleep, particularly during deep slow-wave sleep. Restricting sleep to 5 hours per night for one week reduces testosterone levels by 10–15% in young healthy men. Prioritizing 7–9 hours of quality sleep is among the most impactful interventions for testosterone maintenance.
• Resistance training: Compound weight-bearing exercises — squats, deadlifts, bench press, rows — acutely and chronically elevate testosterone. High-intensity interval training (HIIT) also stimulates testosterone release. Excessive endurance exercise without adequate recovery can paradoxically suppress testosterone via cortisol elevation.
• Body weight management: Adipose tissue, particularly visceral fat, expresses aromatase enzyme that converts testosterone to estradiol. Even modest weight loss (5–10% of body weight) in obese men significantly increases free testosterone.
• Zinc: An essential cofactor for testosterone synthesis and pituitary LH secretion. Deficiency is associated with hypogonadism. Supplementation restores testosterone in zinc-deficient individuals (15–30 mg/day). Food sources include oysters, red meat, pumpkin seeds, and legumes.
• Vitamin D: Functions as a steroid hormone with receptors in Leydig cells. Observational studies consistently show a positive correlation between vitamin D status and testosterone. RCTs in deficient men show modest testosterone improvements with supplementation (2000–4000 IU/day). Optimal 25-OH vitamin D levels are generally above 40–60 ng/mL.
• Stress management: Chronic psychological stress elevates cortisol, which directly inhibits GnRH and LH secretion at the hypothalamic-pituitary axis. Mindfulness, regular relaxation practices, and adequate work-life balance support healthy testosterone levels through the stress-cortisol axis.
• Dietary fat adequacy: Testosterone is synthesized from cholesterol. Excessively low-fat diets (less than 15–20% of calories from fat) are associated with reduced testosterone levels. Healthy fat sources including olive oil, avocados, eggs, and nuts support steroidogenesis.
• Ashwagandha (Withania somnifera): An adaptogenic herb with consistent evidence for modest testosterone elevation and cortisol reduction in stressed individuals. Standard extract doses of 300–600 mg twice daily have shown 14–22% increases in serum testosterone in RCTs.

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References

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