Testosterone
Testosterone is the principal androgen in men, used clinically as replacement therapy for confirmed hypogonadism. It is available as topical gels, injectable esters, transdermal patches and subcutaneous pellets, with selection driven by patient preference and monitoring needs.
- Chemical formula
- C19H28O2
- CAS number
- 58-22-0
- ATC code
- G03BA03
- Molecular weight
- 288.42 g/mol
- Drug class
- Androgen / anabolic steroid
- Also known as
- AndroGel, Testim, Nebido, Sustanon, Testopel
What is it?
Testosterone is the natural androgenic steroid hormone produced primarily by the testes in men and in smaller amounts by the ovaries and adrenal glands in women. It has been used clinically since the 1930s and is on the WHO Essential Medicines List. Modern testosterone replacement therapy (TRT) is approved only for men with biochemically and clinically confirmed hypogonadism. Brand names include AndroGel, Testim and Axiron (gels), Nebido and Sustanon (long-acting injections), Testopel (pellets) and Striant (buccal film), with authorised generics widely available.
Mechanism of action
Testosterone binds to androgen receptors in target tissues, regulating gene expression for male sexual development, libido, erythropoiesis, muscle and bone mass, fat distribution and mood. Some peripheral conversion to dihydrotestosterone (DHT) by 5-alpha reductase amplifies effects in skin, prostate and hair follicles, while conversion to oestradiol by aromatase mediates effects on bone, brain and cardiovascular tissue. Adequate replacement restores physiological androgen signalling.
Pharmacokinetics
Pharmacokinetics depend on formulation. Daily transdermal gels produce relatively stable diurnal levels with serum testosterone in mid-to-high physiological range. Intramuscular cypionate or enanthate every 1–3 weeks produces peak-and-trough fluctuation. Long-acting undecanoate (Nebido) given every 10–14 weeks gives more stable levels. Subcutaneous pellets release for 3–6 months. Selection involves balancing convenience, cost and steady-state physiology.
Indications
Testosterone is approved for primary or secondary hypogonadism in men confirmed by morning total testosterone levels and clinical symptoms. According to current endocrine guidelines, TRT should not be initiated for non-specific symptoms or borderline laboratory values. It is not approved for age-related testosterone decline alone, female sexual dysfunction in most jurisdictions, or athletic performance enhancement. Off-label use in transgender men under specialist supervision is widespread.
Safety profile
Common adverse effects include erythrocytosis (raised haematocrit), acne, oily skin, gynaecomastia, fluid retention and worsening sleep apnoea. Cardiovascular risk has been debated; current evidence suggests no clear net cardiovascular harm in appropriately selected patients. Polycythaemia, prostate-specific antigen rise and breast cancer in women are class concerns. Topical formulations require attention to transfer risk to women and children. Monitoring of haematocrit, PSA, lipids and testosterone levels is recommended at baseline, 3–6 months and annually.
Products containing this ingredient
Frequently asked questions
How is hypogonadism diagnosed before testosterone is started? ▾
Diagnosis requires both clinical symptoms (low libido, fatigue, erectile dysfunction, loss of muscle mass) and at least two morning total testosterone levels below the laboratory reference range. According to endocrine guidelines, TRT should not be started on a single low value or on symptoms alone, because both have non-specific causes. LH and FSH help distinguish primary from secondary hypogonadism.
What lab tests are needed on testosterone? ▾
Total testosterone, haematocrit, PSA (in men over 40), lipid profile and liver function are checked at baseline. After initiation, tests are repeated at 3 and 6 months, then annually. Haematocrit above 54% prompts dose reduction or temporary stop because of polycythaemia risk. PSA monitoring screens for unmasked prostate disease, not for new cancer caused by treatment.
Which testosterone formulation is best? ▾
There is no universally best formulation. Daily gels produce stable levels but require attention to transfer risk; weekly intramuscular injections are inexpensive but produce peak-and-trough swings; long-acting undecanoate or pellets give the most stable levels but require clinic visits. According to current practice, choice is driven by patient preference, monitoring availability and cost.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.