Testosterone is the most frequently ordered sex hormone test in clinical practice worldwide, yet its interpretation is more nuanced than a single number suggests. The biologically meaningful fraction — free testosterone — accounts for only 1–4% of the total circulating amount. The rest is protein-bound and inactive. For IVD developers designing testosterone assay platforms, and for clinicians interpreting results, understanding the distinctions between total, free, and bioavailable testosterone, and how normal ranges shift across age and sex, is foundational. This guide provides structured reference data, clinical interpretation principles, and a concise view of the testing methods used in modern diagnostic laboratories.
What Is Testosterone?
Testosterone is an androgen — a steroid hormone synthesized primarily in the Leydig cells of the testes in men, and in the ovaries and adrenal cortex in women. It belongs to the C19 steroid family and is derived from cholesterol via the pregnenolone pathway, sharing biosynthetic intermediates with estradiol and hormone balance throughout the reproductive axis.
Testosterone exerts its effects by binding to androgen receptors (AR) expressed in muscle, bone, brain, liver, skin, and reproductive tissues. In men, it drives spermatogenesis, secondary sex characteristics, and anabolic metabolism. In women, it contributes to libido, bone maintenance, and muscle tone at physiological concentrations — effects that become pathological when testosterone is elevated above the normal female range.
In circulation, testosterone exists in three forms:
- SHBG-bound (~44–65%): Tightly bound to sex hormone-binding globulin; biologically inactive and unable to enter cells.
- Albumin-bound (~30–44%): Loosely bound; dissociates readily in capillary beds and is considered bioavailable.
- Free (~1–4%): Unbound; immediately active and the fraction most directly correlated with androgenic effects.
The sum of free and albumin-bound fractions is termed bioavailable testosterone — the clinically relevant pool when assessing androgen status in conditions where SHBG is abnormal.
Normal Testosterone Levels in Men
Normal testosterone concentrations in men decline progressively with age. Reference intervals established by the Endocrine Society and validated in population studies such as the Framingham Heart Study and NHANES define the following ranges for morning fasting specimens. All values reflect serum immunoassay results calibrated to mass spectrometry (LC-MS/MS) reference methods.
| Age Group | Total Testosterone (ng/dL) | Free Testosterone (ng/dL) | Bioavailable T (ng/dL) |
|---|---|---|---|
| 20–30 years | 600–1,000 | 9.0–30.0 | 175–460 |
| 30–40 years | 500–900 | 8.0–26.0 | 150–420 |
| 40–50 years | 400–800 | 7.0–22.0 | 130–370 |
| 50–60 years | 350–700 | 6.0–19.0 | 110–310 |
| 60–70 years | 300–650 | 5.0–17.0 | 90–280 |
| 70+ years | 250–600 | 4.0–14.0 | 70–230 |
Clinical note: The Endocrine Society defines male hypogonadism as a consistently low morning total testosterone below 300 ng/dL with supporting symptoms — not a single measurement. Two independent morning draws are required before diagnosis. (Source: Bhasin et al., JCEM 2018)
Total testosterone alone can be misleading in patients with altered SHBG. Obesity, hypothyroidism, and ageing all raise SHBG, reducing free testosterone even when total testosterone reads within the reference interval. For these patients, free or bioavailable testosterone measurement provides a more accurate picture of androgenic status.
Normal Testosterone Levels in Women
Testosterone concentrations in women are approximately 10–20× lower than in men and are influenced by menstrual cycle phase, age, and menopausal status. The values below reflect total and free testosterone from morning specimens using LC-MS/MS calibrated methods, which offer superior accuracy at the low concentrations typical in female specimens.
| Cycle Phase / Status | Total Testosterone (ng/dL) | Free Testosterone (pg/mL) |
|---|---|---|
| Follicular phase | 15–70 | 1.5–8.5 |
| Ovulatory peak | 20–80 | 2.0–9.5 |
| Luteal phase | 15–70 | 1.5–8.5 |
| Postmenopause | 7–40 | <5.0 |
| Pregnancy (third trimester) | up to 3× pre-pregnancy | variable |
In women, testosterone is produced roughly equally by the ovaries and adrenal glands, with a small contribution from peripheral conversion of DHEAS. After menopause, ovarian androgen production falls, but adrenal contribution continues — so postmenopausal women typically retain 40–50% of their premenopausal testosterone level, unlike the near-total oestrogen loss that drives vasomotor symptoms. This distinction is clinically important for interpreting hormone panels alongside estradiol and hormone balance markers.
Low Testosterone in Men — Symptoms & Causes
Low testosterone, clinically termed hypogonadism or informally "low T," affects an estimated 2–4% of men under 40, rising to 30% in men over 70. The clinical presentation spans sexual, physical, and metabolic domains.
Symptoms of Low Testosterone in Men
- Sexual: Reduced libido, erectile dysfunction, decreased ejaculatory volume, reduced morning erections
- Physical: Decreased muscle mass and strength, increased central adiposity, reduced bone mineral density, decreased body and facial hair
- Mood and cognition: Fatigue, depressed mood, poor concentration, sleep disturbance, reduced motivation and drive
- Metabolic: Insulin resistance, dyslipidaemia (elevated LDL, reduced HDL), increased visceral fat mass
Causes of Low Testosterone
Low testosterone results from failure at one of three levels of the hypothalamic–pituitary–gonadal (HPG) axis:
- Primary hypogonadism (testicular failure): Klinefelter syndrome (47,XXY), orchitis, testicular torsion, chemotherapy or radiation damage, bilateral orchidectomy. LH and FSH are elevated (compensatory pituitary drive); testosterone is low.
- Secondary hypogonadism (pituitary/hypothalamic dysfunction): Hyperprolactinaemia, pituitary adenoma, haemochromatosis, Kallmann syndrome, opioid use, glucocorticoid excess. LH and FSH are low or inappropriately normal alongside low testosterone.
- Late-onset hypogonadism (functional/age-related): The most prevalent form, driven by progressive Leydig cell decline, rising SHBG, and reduced HPG axis pulsatility. Obesity amplifies the effect through aromatase-driven conversion of testosterone to oestradiol in adipose tissue.
Diagnostic trap: Symptoms of low testosterone overlap substantially with depression, sleep apnoea, hypothyroidism, and anaemia. Biochemical confirmation is mandatory — treating non-hypogonadal men based on symptoms alone exposes them to unnecessary risks. Always measure two morning total testosterone levels before pursuing further workup.
For information on low testosterone treatment options, including an evidence-based review of androgen replacement approaches, see our dedicated clinical guide.
High Testosterone in Women — Causes & PCOS
Elevated testosterone in women — defined as total testosterone consistently above 70–80 ng/dL or free testosterone above 8.5 pg/mL — is clinically termed hyperandrogenaemia and produces a characteristic symptom cluster driven by androgen receptor activation in androgen-sensitive tissues.
Clinical Signs of High Testosterone in Women
- Skin and hair: Acne (particularly jaw, chin, chest), hirsutism (excess terminal hair on face, abdomen, inner thighs), androgenic alopecia (male-pattern hair thinning)
- Menstrual cycle: Oligomenorrhoea (cycles >35 days apart), amenorrhoea, anovulation
- Metabolic: Insulin resistance, central weight gain, elevated triglycerides
- Reproductive: Reduced fertility, recurrent miscarriage (associated with PCOS)
- Severe hyperandrogenaemia: Virilisation — clitoromegaly, voice deepening — suggests androgen-secreting tumour rather than PCOS
Causes of High Testosterone in Women
The differential diagnosis of female hyperandrogenaemia, in order of prevalence:
- Polycystic ovary syndrome (PCOS): Accounts for 70–80% of hyperandrogenaemia cases. Defined by the Rotterdam criteria (2003) as ≥2 of: oligo-ovulation, biochemical or clinical hyperandrogenaemia, polycystic ovarian morphology. Elevated LH:FSH ratio and raised total or free testosterone are characteristic biochemical findings.
- Non-classical congenital adrenal hyperplasia (NCAH): 21-hydroxylase deficiency presenting in adulthood; elevated 17-hydroxyprogesterone distinguishes from PCOS.
- Androgen-secreting tumours: Rare; adrenal or ovarian origin. Suggested by rapidly progressive virilisation and testosterone >150 ng/dL. Requires imaging and DHEAS measurement.
- Cushing syndrome: Cortisol excess drives adrenal androgen overproduction; confirmed by 24-hour urine free cortisol or late-night salivary cortisol.
- Exogenous androgen exposure: Anabolic steroid use, DHEA supplementation, topical testosterone cross-contamination.
The role of progesterone and estrogen in the hormonal context of PCOS is important — low progesterone in the luteal phase confirms anovulation, while the oestradiol profile helps distinguish ovarian from adrenal androgen sources.
Sekbio supplies monoclonal antibody pairs for testosterone immunoassay development, calibrated for IVD performance at both male (300–1,000 ng/dL) and female (<80 ng/dL) concentration ranges. Request technical specifications →
How Is Testosterone Tested?
Testosterone measurement in clinical and IVD settings spans several technologies, each with distinct tradeoffs in accuracy, throughput, and applicable concentration range.
Immunoassay (CLIA / ECLIA)
Automated chemiluminescent immunoassays (CLIA) and electrochemiluminescent immunoassays (ECLIA) on platforms such as Roche Cobas, Abbott Architect, and Siemens Atellica are the dominant method in routine clinical laboratories worldwide. They offer high throughput (>200 samples/hour), low per-test cost, and fully automated operation. For total testosterone measurement in men — where concentrations typically exceed 300 ng/dL — immunoassay performance is generally adequate, with CV <10% between lots from quality suppliers.
The limitation is specificity at low concentrations. Testosterone immunoassays use anti-testosterone monoclonal antibodies that can show cross-reactivity with structurally related steroids — androstenedione, DHEAS, dihydrotestosterone (DHT) — at levels that become significant when total testosterone is below 50 ng/dL, as in women and children. The published cross-reactivity of IVD-grade testosterone antibody pairs must be <5% for DHT and <1% for DHEAS to meet clinical specificity requirements at female concentrations. (Source: Vermeulen et al., JCEM 1999 — reference equilibrium dialysis method)
LC-MS/MS (Gold Standard)
Liquid chromatography–tandem mass spectrometry offers superior accuracy and specificity across the full testosterone concentration range. It directly measures testosterone molecular mass and distinguishes it from cross-reactive steroids with near-zero interference. LC-MS/MS is the reference method recommended by CLSI and the Endocrine Society for low-concentration specimens (women, children, post-treatment monitoring). It is the calibration anchor for all immunoassay reference intervals cited in this article.
Free Testosterone Measurement
Equilibrium dialysis (ED) is the gold-standard method for free testosterone — the sample is dialysed across a semi-permeable membrane, separating bound from unbound fractions. The practical alternative is calculated free testosterone using total testosterone, SHBG, and albumin concentration via the Vermeulen or Ly equations. Both correlate well with ED when SHBG is within the normal range; calculated free T diverges from ED in extreme SHBG conditions (severe obesity, liver disease).
Quick Reference: Testosterone Testing Methods
| Method | Best Use Case | Limitation |
|---|---|---|
| CLIA/ECLIA immunoassay | Male screening, high throughput | Cross-reactivity <50 ng/dL |
| LC-MS/MS | Women, children, confirmatory | High cost, slower turnaround |
| Equilibrium dialysis | Free T (reference) | Manual, low throughput |
| Calculated free T | Free T (routine clinical) | Requires SHBG measurement |
Testosterone Antibody Pairs for IVD Development
Sekbio supplies anti-testosterone monoclonal antibody pairs with <5% DHT cross-reactivity, validated for CLIA and lateral flow platforms across male and female concentration ranges.
Request Technical Datasheet Browse All IVD AntibodiesFrequently Asked Questions About Testosterone
What is a dangerously low testosterone level in men?
There is no single universally "dangerous" cutoff, but the Endocrine Society defines male hypogonadism as a consistently low morning total testosterone below 300 ng/dL (10.4 nmol/L) combined with symptoms such as fatigue, reduced libido, or loss of muscle mass. Levels below 200 ng/dL are associated with more pronounced symptoms and a higher risk of metabolic complications including insulin resistance and reduced bone density.
A single low result is not sufficient — at least two morning measurements on separate days are required for diagnosis, as testosterone follows a diurnal pattern with peak values 20–30% higher in the morning than the evening.
What is considered high testosterone in women?
In adult women of reproductive age, total testosterone above 70–80 ng/dL is generally considered elevated. Free testosterone above 8.5 pg/mL or DHEAS above 350 µg/dL in women under 50 warrants clinical investigation.
The most common cause is polycystic ovary syndrome (PCOS), which affects 5–10% of women of reproductive age and is characterised by elevated androgens, oligo-ovulation, and polycystic ovarian morphology on ultrasound. Other causes include congenital adrenal hyperplasia, androgen-secreting tumours, and exogenous androgen exposure.
What is the difference between total testosterone and free testosterone?
Total testosterone measures the entire amount of testosterone in the blood — the sum of protein-bound and unbound fractions. Approximately 44–65% is bound tightly to sex hormone-binding globulin (SHBG), 30–44% is loosely bound to albumin, and only 1–4% circulates as free (unbound) testosterone.
Free testosterone is the biologically active fraction that enters cells and exerts androgenic effects. In conditions where SHBG is elevated (obesity, thyroid disease, ageing), total testosterone may appear normal while free testosterone is actually low, explaining symptoms. Free testosterone measurement by equilibrium dialysis is considered the gold standard; calculated free testosterone using total T, SHBG, and albumin is the practical clinical alternative.
What are the symptoms of low testosterone in men?
Low testosterone (low T or hypogonadism) in men produces a characteristic cluster of symptoms:
- Sexual: Reduced libido, erectile dysfunction, decreased ejaculatory volume, reduced morning erections
- Physical: Reduced muscle mass, increased central adiposity, decreased body and facial hair, reduced bone density
- Mood and cognition: Fatigue, depressed mood, poor concentration, reduced motivation, sleep disturbance
- Metabolic: Insulin resistance, dyslipidaemia, increased visceral fat mass
Not all symptoms are specific to testosterone deficiency — thyroid dysfunction, depression, and sleep apnoea can produce similar presentations. Diagnosis requires biochemical confirmation with two low morning testosterone measurements alongside appropriate symptoms.
Why does testosterone decline with age in men?
Total testosterone in men declines at approximately 1–2% per year from age 30 onwards due to two parallel mechanisms. First, testicular Leydig cell mass and function decrease progressively with age, reducing the gonadal response to LH stimulation. Second, the pulsatile release of LH from the pituitary becomes less regular, further impairing the hypothalamic-pituitary-gonadal (HPG) axis drive.
Concurrently, SHBG levels rise with age, binding a greater proportion of circulating testosterone and further reducing the bioavailable and free fractions. By age 70, approximately 30% of men have total testosterone below 300 ng/dL, compared to less than 5% of men under 40.
What method is used to measure testosterone in clinical labs?
Most clinical laboratories use automated immunoassay platforms (CLIA/ECLIA) for total testosterone. These offer high throughput and low cost, and perform adequately for male-range concentrations (>300 ng/dL). However, immunoassays can show cross-reactivity with testosterone precursors at concentrations below 50 ng/dL — relevant for female and paediatric specimens.
The gold standard is liquid chromatography–tandem mass spectrometry (LC-MS/MS), which directly measures testosterone molecular mass and is recommended by the Endocrine Society for low-concentration measurement. All reference intervals cited in this article are calibrated to LC-MS/MS values.
How do testosterone levels differ between morning and evening?
Testosterone exhibits a clear circadian rhythm, with peak concentrations occurring between 7–10 AM and trough values 20–30% lower in the late afternoon and evening. This pattern is most pronounced in younger men and becomes blunted with age.
All clinical testosterone measurements should be drawn as morning fasting samples, ideally between 7–10 AM. Drawing samples in the afternoon in a symptomatic patient can produce an artificially low result that mimics hypogonadism. IVD testosterone assay developers should specify collection time in their kit instructions and calibrate reference intervals using morning specimens.