Testosterone

Mechanism of action

◎Androgen receptor (AR), a nuclear receptorTestosterone

ⓘ The figure shows testosterone itself in the ligand-binding domain of the human androgen receptor.

Testosterone binds the androgen receptor directly and, after conversion by 5-α-reductase, as dihydrotestosterone; part is aromatised to estradiol. In hypogonadal men it restores libido, erections, secondary sexual characteristics, muscle, bone density and red-cell production. Through negative feedback it suppresses LH and FSH, so the testes make less of their own testosterone and sperm.

1. Testosterone reaches target tissues and enters the cells
2. It binds the androgen receptor directly or as DHT
3. Androgen-dependent genes switch on in muscle, bone, skin and genitals
4. LH and FSH fall, lowering sperm production

Indications

• Primary hypogonadism (testicular failure: e.g. cryptorchidism, orchidectomy, Klinefelter syndrome) Male hypogonadism →
• Testicular failure in Klinefelter syndrome Klinefelter syndrome (47,XXY) →
• Hypogonadotropic hypogonadism (pituitary or hypothalamic deficiency) Male hypogonadism →
• Delayed puberty in boys

Adverse effects

Common

• Acne and oily skin
• Injection-site pain
• Gynaecomastia
• Mood swings, sleep problems
• Fluid retention and weight gain

Contraindications

• Breast cancer in men
• Known or suspected prostate cancer
• Pregnancy (it virilises a female fetus)
• Serious cardiac, liver or kidney disease
• Hypersensitivity to the drug

Key interactions

Oral anticoagulants (warfarin)

Greater anticoagulant sensitivity; INR is watched and the anticoagulant dose may need reducing

Insulin and other antidiabetics

Insulin requirements may fall in people with diabetes

What is monitored

• Haemoglobin and haematocrit
• Testosterone levels
• Blood pressure
• Prostate symptoms (and PSA as clinically indicated)
• Lipids and liver tests; bone age in boys with delayed puberty

Administration and safety

• The cypionate and enanthate injections are deep intramuscular (gluteal) and never intravenous
• Some products are given only in a health facility with observation afterwards because of a risk of breathing problems or allergy
• Injectable, gel and patch products are not interchangeable without an order
• Testosterone is a controlled substance and is not shared

Pregnancy and breastfeeding

Contraindicated in pregnancy: it can virilise a female fetus.

Nursing care

• Check haematocrit and blood pressure before and during treatment
• Teach to report leg pain or swelling, chest pain or sudden breathlessness
• Ask about urinary symptoms and sleep apnoea
• Explain that it lowers sperm count and is not a fertility treatment
• Teach safe storage and that it must not be shared or used at higher doses

Pharmacokinetics

Absorption

Injected as long-acting esters into muscle (some under the skin), or absorbed through the skin from gels and patches

Distribution

Mostly bound to SHBG and albumin

Metabolism

In the liver; converted in tissues to DHT and estradiol

Elimination

As conjugated metabolites in urine

Test yourself

Think of the answer, then tap the question to see it.

Add to my cards (3)

More Pharmacology questions →

Connections

← Back to the class: Androgens

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - TESTOSTERONE CYPIONATE injection (DailyMed (U.S. National Library of Medicine), 2026); Testosterone Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Send

Representative model, for educational purposes only.