Testosterone
Primary male sex hormone for muscle building, libido, and energy
Also known as
Definition
Testosterone is the primary androgen, produced in the testes (men), ovaries, and adrenal glands (women). It is responsible for muscle building, bone density, libido, mood, energy metabolism, and erythropoiesis. Total testosterone encompasses free testosterone (~2–3%), albumin-bound (~38%), and SHBG-bound (~60%). Only free and albumin-bound testosterone is biologically active (= bioavailable testosterone).
| Parameter | Value |
|---|---|
| Unit | nmol/l (ng/dl) |
| Reference Range | Men: 9.9–27.8 nmol/l (286–800 ng/dl) | Women: 0.29–1.67 nmol/l (8–48 ng/dl) |
| Optimal Range (lab2go) | Men: 18–27 nmol/l (520–780 ng/dl) (lab2go Optimum Zone — upper third of reference range for vitality) | Women: 0.7–1.4 nmol/l (20–40 ng/dl) |
Conversion: 1 nmol/l = 28.8 ng/dl. US labs use ng/dl (men: 270–1070 ng/dl).
↓ What a low value means
Hypogonadism (men: < 9.9 nmol/l; women: < 0.29 nmol/l) can be primary (testicular/ovarian failure) or secondary (pituitary/hypothalamic). Symptoms: loss of libido, erectile dysfunction, muscle wasting, bone density loss, anemia, depression, fatigue, increased body fat. Testosterone in men declines ~1–2% per year from age 30.
↑ What a high value means
Elevated values in women (> 1.67 nmol/l) can indicate PCOS, congenital adrenal hyperplasia, or androgen-producing tumors. In men: anabolic steroids, testosterone therapy, or rarely a Leydig cell tumor. Symptoms in women: hirsutism, acne, menstrual irregularities, clitoral hypertrophy.
✓ How to optimize this marker
Naturally: optimize sleep (testosterone is mainly produced at night; 7–9 hours raises testosterone by ~15–20%), resistance training, weight loss with overweight (each 10 kg of fat loss raises testosterone ~1–2 nmol/l), ensure adequate zinc, optimize vitamin D (> 40 ng/ml), reduce stress.
When to test
Morning between 7–10 AM (circadian peak), fasting. With symptoms such as loss of libido, fatigue, muscle loss, or infertility. Always measure SHBG simultaneously, as SHBG changes strongly influence the free fraction. Calculate or directly measure free testosterone.
Frequently asked questions
What is the difference between total testosterone and free testosterone? +
Total testosterone measures all fractions: free (2–3%), albumin-bound (~38%), and SHBG-bound (~60%). Only free and albumin-bound testosterone is biologically active. High SHBG (from liver cirrhosis, hyperthyroidism, estrogen, aging) can produce low free testosterone and hypogonadism symptoms despite normal total testosterone.
How much does testosterone decline with age? +
In men, total testosterone declines ~1–2% per year from age 30 (EMAS study). Free testosterone declines faster (~2–3%/year) as SHBG rises with age. By age 70, the average man has 30–40% less testosterone than at 30. This decline is variable and highly dependent on lifestyle.
Can women also have testosterone deficiency? +
Yes. Women produce testosterone in the ovaries and adrenal glands; it is important for libido, muscle strength, and bone density. After menopause or oophorectomy, testosterone declines markedly. Symptoms such as loss of libido, fatigue, and muscle weakness in women should prompt consideration of testosterone deficiency.
When is testosterone replacement therapy appropriate? +
The Endocrine Society recommends TRT in men with confirmed hypogonadism (two morning measurements < 10 nmol/l) AND clinical symptoms. A low value alone without symptoms, or symptoms without a low value, does not justify therapy. Discuss fertility planning beforehand, as TRT suppresses spermatogenesis.
Sources
- Bhasin S et al. (2018) Testosterone Therapy in Men with Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab.
- Wu FCW et al. (2010) Identification of late-onset hypogonadism in middle-aged and elderly men (EMAS). N Engl J Med.
- Travison TG et al. (2007) A population-level decline in serum testosterone levels in American men. J Clin Endocrinol Metab.
Last Reviewed: May 28, 2026 · sina
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.