Pharmacology of Testosterone

1. Introduction/Overview

Testosterone, the principal endogenous androgen, serves as a critical hormone in mammalian physiology, orchestrating a wide array of developmental and homeostatic functions. As a steroidal compound derived from cholesterol, its pharmacological significance extends far beyond its natural endocrine role. The clinical application of exogenous testosterone and its synthetic analogs represents a cornerstone in the management of hypogonadism and certain other medical conditions. Understanding its pharmacology is essential for the rational and safe use of androgen therapies, which carry substantial therapeutic benefits alongside significant risks of misuse and adverse effects.

The clinical relevance of testosterone pharmacology is considerable, given the increasing diagnosis of testosterone deficiency syndromes and the expanding, though often controversial, exploration of its use in aging males. Furthermore, the illicit use of anabolic-androgenic steroids (AAS) for performance and image enhancement presents a persistent public health challenge, necessitating that healthcare professionals possess a thorough understanding of their effects. The importance of this topic is underscored by the need to differentiate between evidence-based therapeutic applications and unsubstantiated or dangerous off-label use.

Learning Objectives

• Describe the biochemical synthesis, chemical classification, and structural analogs of testosterone.
• Explain the molecular mechanism of action of testosterone, including its conversion to active metabolites and genomic versus non-genomic signaling pathways.
• Analyze the pharmacokinetic profile of testosterone, including the rationale behind various administration routes and formulation designs.
• Evaluate the approved therapeutic indications for testosterone therapy and critically appraise common off-label uses.
• Identify major adverse effects, contraindications, and drug interactions associated with testosterone and anabolic-androgenic steroid use.

2. Classification

Testosterone and its related compounds are classified primarily as androgens or anabolic-androgenic steroids (AAS). This classification is based on their physiological and pharmacological actions, although the distinction between “androgenic” (masculinizing) and “anabolic” (tissue-building) effects is largely artificial, as both are mediated through the same receptor.

Chemical and Pharmacological Classification

Chemically, testosterone is a steroid hormone, specifically a 19-carbon steroid derived from cyclopentanoperhydrophenanthrene. Its core structure is androstane, making it a C₁₉ steroid with a hydroxyl group at the 17β position and a ketone group at the 3 position (17β-hydroxyandrost-4-en-3-one). This structure is fundamental to its receptor binding.

Pharmacologically, these agents can be categorized as follows:

• Endogenous Androgens: Testosterone and its more potent metabolite, dihydrotestosterone (DHT).
• Exogenous Testosterone Esters: These are prodrugs created by esterifying the 17β-hydroxyl group of testosterone with various fatty acids (e.g., enanthate, cypionate, propionate). The esterification increases lipid solubility, allowing for depot formulations with prolonged release from intramuscular injection sites. The ester is cleaved by esterases in vivo to release active testosterone.
• 17α-Alkylated Androgens: Synthetic analogs (e.g., methyltestosterone, oxandrolone, stanozolol) modified with an alkyl group at the 17α position. This modification confers oral bioavailability by reducing first-pass hepatic metabolism but is also associated with increased hepatotoxicity.
• Non-alkylated Oral Androgens: Testosterone undecanoate, formulated in oil-filled capsules, is absorbed via the lymphatic system, bypassing first-pass hepatic metabolism and thereby reducing liver strain.
• Transdermal and Mucosal Delivery Systems: Gels, patches, and buccal systems designed to deliver unmodified testosterone, aiming to mimic physiological circadian rhythms and avoid the peaks and troughs associated with injectable esters.

3. Mechanism of Action

The pharmacological effects of testosterone are mediated primarily through its interaction with the androgen receptor (AR), a member of the nuclear receptor superfamily. The mechanism involves both classical genomic pathways and faster, non-genomic signaling.

Receptor Interactions and Genomic Pathways

Testosterone itself is a prohormone in many tissues. Its action requires activation via one of two major pathways:

1. Conversion to Dihydrotestosterone (DHT): In peripheral tissues such as the prostate, skin, and hair follicles, the enzyme 5α-reductase irreversibly reduces testosterone to DHT. DHT has a 2- to 10-fold higher affinity for the AR than testosterone and forms a more stable complex with the receptor.
2. Direct Binding: In other tissues like skeletal muscle and bone, testosterone can bind directly to the AR with sufficient potency.

Upon entering the target cell, testosterone or DHT binds to the cytosolic AR. This binding induces a conformational change, receptor dimerization, dissociation from heat shock proteins, and translocation into the nucleus. The hormone-receptor complex then binds to specific DNA sequences known as androgen response elements (AREs) in the promoter regions of target genes. This recruitment facilitates the assembly of transcriptional co-regulators, leading to either upregulation or downregulation of gene transcription. The subsequent synthesis of new proteins (e.g., growth factors, structural proteins) accounts for the delayed, sustained effects of androgens on tissue growth and function.

Non-Genomic Mechanisms

Rapid effects of testosterone, observable within seconds to minutes, are believed to occur through non-genomic pathways. These may involve interaction with membrane-associated ARs or other receptors, such as G-protein-coupled receptors. This signaling can lead to rapid activation of second messenger systems, including increases in intracellular calcium and activation of protein kinase pathways like MAPK/ERK and PI3K/Akt. These pathways may contribute to vasodilation, neuro-modulation, and acute effects on cell signaling.

Physiological and Pharmacological Effects

• Anabolic Effects: Promotion of nitrogen retention and protein synthesis in skeletal muscle, leading to increased muscle mass and strength. Stimulation of erythropoiesis via increased erythropoietin production and direct effects on bone marrow stem cells.
• Androgenic Effects: Development and maintenance of male secondary sexual characteristics (facial/body hair, deep voice, libido), spermatogenesis, and prostate growth.
• Metabolic Effects: Modulation of lipid metabolism (often decreasing HDL cholesterol), promotion of insulin sensitivity in some contexts, and influence on bone mineral density via stimulation of osteoblast activity.
• Central Nervous System Effects: Influence on mood, aggression, and cognitive functions through ARs widely distributed in the brain.

4. Pharmacokinetics

The pharmacokinetics of testosterone are highly dependent on the route of administration and chemical formulation, which are designed to overcome its inherent challenges: poor oral bioavailability due to extensive first-pass metabolism and short plasma half-life in its native form.

Absorption

• Oral Unmodified Testosterone: Rapidly and extensively metabolized by first-pass hepatic metabolism (via CYP3A4 and others), resulting in bioavailability of less than 5%, making it clinically impractical.
• Oral 17α-Alkylated Derivatives: The alkyl group at the 17α position impedes hepatic breakdown, allowing for significant oral bioavailability (e.g., 50-100% for oxandrolone).
• Oral Testosterone Undecanoate: Absorbed with dietary fats into the lymphatic system via the thoracic duct, thereby bypassing the portal circulation and first-pass hepatic metabolism. Bioavailability is variable and food-dependent.
• Intramuscular Esters (e.g., enanthate, cypionate): Injected as oil-based depot formulations. Absorption is slow and rate-limited by the hydrolysis of the ester bond at the injection site and in plasma. This results in prolonged action with dosing intervals of 1 to 4 weeks, but produces supraphysiological peaks and subphysiological troughs in serum concentration.
• Transdermal Systems (Gels, Patches): Provide continuous delivery through the skin. Steady-state concentrations are typically achieved within 24-72 hours. Bioavailability ranges from 5-15% for gels, depending on application site and skin characteristics. Risk of secondary exposure exists with gels.
• Buccal and Nasal Systems: Deliver testosterone across mucous membranes, offering a non-invasive route with twice-daily dosing to mimic diurnal rhythms.
• Subcutaneous Pellets: Crystalline testosterone pellets implanted subcutaneously provide stable release for 3 to 6 months.

Distribution

In circulation, approximately 98% of testosterone is bound to plasma proteins. The majority (about 65%) is tightly bound to sex hormone-binding globulin (SHBG) with high affinity, while roughly 33% is loosely bound to albumin. Only 1-3% circulates as free, biologically active hormone. The distribution volume is relatively large, approximately 1.0 L/kg. Testosterone and its analogs readily cross the blood-brain barrier and the placenta.

Metabolism

Hepatic metabolism is the primary route of inactivation. Major pathways include:

1. Oxidation: Catalyzed by hepatic cytochrome P450 enzymes (notably CYP3A4), converting testosterone to androstenedione.
2. Reduction:
   • Via 5α-reductase (types I and II) to form the potent androgen dihydrotestosterone (DHT).
   • Via 5β-reductase to form etiocholanolone, an inactive metabolite.
3. Conjugation: The oxidized and reduced metabolites are conjugated with glucuronic acid or sulfate in the liver, rendering them water-soluble for renal excretion.

The 17α-alkylated compounds undergo slower hepatic oxidation, contributing to their oral activity but also to their increased potential for hepatotoxicity, including cholestasis and peliosis hepatis.

Excretion

Conjugated metabolites (primarily glucuronides and sulfates) are excreted mainly in urine (approximately 90%), with a smaller fraction eliminated in bile and feces. The elimination half-life of endogenous testosterone is short, approximately 10 to 100 minutes. For exogenous formulations, the effective half-life is determined by the release rate from the depot: transdermal systems have a half-life of about 1-2 hours after removal, while intramuscular esters have apparent half-lives ranging from 4-8 days (cypionate/enanthate) to weeks (undecanoate).

Key pharmacokinetic parameters for common formulations illustrate these differences. For example, intramuscular testosterone cypionate typically achieves a peak concentration (C_max) 24-72 hours post-injection, with an elimination rate constant (k_el) leading to a terminal half-life (t_1/2) of approximately 8 days. The area under the curve (AUC) is proportional to dose for most formulations within the therapeutic range, following linear kinetics: AUC = Dose ÷ Clearance.

5. Therapeutic Uses/Clinical Applications

Therapeutic use of testosterone is indicated primarily for replacement therapy in states of documented deficiency. Its application should be guided by confirmed hypogonadism and clinical symptoms, not by serum levels alone.

Approved Indications

• Male Hypogonadism (Primary and Secondary): This is the principal indication. Primary hypogonadism (hypergonadotropic) results from testicular failure (e.g., Klinefelter syndrome, orchitis, chemotherapy). Secondary hypogonadism (hypogonadotropic) results from hypothalamic-pituitary dysfunction (e.g., pituitary tumors, idiopathic hypogonadotropic hypogonadism). Therapy aims to restore serum testosterone to the mid-normal range and alleviate symptoms such as low libido, erectile dysfunction, fatigue, decreased muscle mass, and osteopenia.
• Delayed Male Puberty: Used cautiously to induce puberty in boys with constitutional delay, typically with low doses to avoid premature epiphyseal closure.
• Metastatic Breast Cancer in Women: Historical use for its anti-estrogenic effects in hormone-responsive advanced breast cancer, though largely superseded by other endocrine therapies.
• Specific Formulation Indications: Testosterone undecanoate injection is approved specifically for male hypogonadism; buccal testosterone is indicated for replacement therapy in men.

Off-Label and Investigational Uses

Several off-label uses are common, though their evidence base and risk-benefit profiles vary considerably.

• Age-Related Hypogonadism (“Andropause”): Controversial and not universally approved. Treatment may be considered in older men with consistent symptoms and unequivocally low serum levels, but cardiovascular risks require careful evaluation.
• Wasting Syndromes: Used to counteract cachexia associated with chronic diseases like HIV/AIDS and cancer, aiming to improve lean body mass, strength, and quality of life.
• Anemia of Chronic Disease: Particularly in patients with renal failure, where it can stimulate erythropoiesis and reduce requirements for erythropoiesis-stimulating agents.
• Osteoporosis in Hypogonadal Men: To increase bone mineral density.
• Gender-Affirming Hormone Therapy: A cornerstone of masculinizing hormone therapy for transgender men or non-binary individuals, administered to induce and maintain male secondary sex characteristics.

The use of anabolic steroids for performance enhancement or bodybuilding is not a therapeutic indication and is associated with significant health risks.

6. Adverse Effects

Adverse effects of testosterone therapy are often dose-dependent and related to its pharmacological actions on various organ systems. The risk profile differs between physiological replacement and supraphysiological dosing.

Common Side Effects

• Androgenic Effects: Acne, oily skin, increased facial/body hair (hirsutism), male-pattern baldness in genetically predisposed individuals, and increased sebaceous gland activity.
• Reproductive System: Suppression of the hypothalamic-pituitary-gonadal (HPG) axis via negative feedback, leading to reduced luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion. This can cause testicular atrophy, oligospermia, or azoospermia, and reduced fertility. Priapism (prolonged, painful erection) is a rare but serious urological emergency.
• Fluid and Electrolyte Effects: Mild sodium and water retention, which can lead to edema, weight gain, and exacerbation of pre-existing hypertension or congestive heart failure.
• Hematologic: Dose-related stimulation of erythropoiesis, which can progress to polycythemia (elevated hematocrit), increasing the risk of thromboembolic events, especially in older men.

Serious and Rare Adverse Reactions

• Cardiovascular Risk: A subject of ongoing debate and research. Potential risks may include increased blood pressure, adverse changes in lipid profile (decreased HDL-C, increased LDL-C), promotion of atherosclerosis, and a possible increased risk of major adverse cardiac events (MACE) in vulnerable populations.
• Hepatotoxicity: Primarily associated with 17α-alkylated oral androgens. Effects range from reversible elevations in liver transaminases to cholestatic jaundice, peliosis hepatis (blood-filled cysts in the liver), and hepatocellular adenoma or carcinoma with long-term use.
• Psychiatric and Behavioral: May include increased aggression, irritability, mood swings, euphoria, and dependence. Severe cases can involve manic episodes or exacerbation of underlying psychiatric disorders.
• Endocrine: Gynecomastia can occur due to peripheral aromatization of testosterone to estradiol. In children, premature epiphyseal closure leads to stunted growth.
• Prostatic Effects: May cause benign prostatic hyperplasia (BPH) worsening (e.g., increased urinary symptoms) and potentially stimulate the growth of subclinical prostate cancer. It is contraindicated in men with active prostate or breast cancer.

Black Box Warnings

Testosterone products carry several boxed warnings mandated by regulatory agencies:

• Increased Risk of Major Adverse Cardiovascular Events (MACE): Some studies have suggested an increased risk of heart attack, stroke, and cardiovascular death, particularly in older men and those with pre-existing heart disease. The overall evidence remains controversial, but the warning prompts careful cardiovascular risk assessment before and during therapy.
• Potential for Abuse: Testosterone and other AAS have a high potential for abuse, often at doses far exceeding therapeutic levels, leading to serious psychiatric and physical dependence.
• Secondary Exposure to Children and Women (Transdermal Gels): Accidental transfer of testosterone gel from a patient’s skin to children or women can result in virilization (e.g., inappropriate enlargement of the genitalia, advanced bone age in children, increased body hair in women). Strict application and hygiene instructions are critical.

7. Drug Interactions

Testosterone can interact with several classes of medications, altering its own efficacy or the effects of co-administered drugs.

Major Drug-Drug Interactions

• Anticoagulants (Warfarin): Testosterone may potentiate the anticoagulant effect by reducing the synthesis of clotting factors and increasing fibrinolysis. Prothrombin time (PT/INR) requires close monitoring, and warfarin dosage may need reduction.
• Corticosteroids and ACTH: Concurrent use may exacerbate fluid retention and edema.
• Insulin and Oral Hypoglycemics: Androgens may enhance hypoglycemic effects, potentially necessitating a reduction in antidiabetic medication dose.
• Hepatotoxic Drugs: Concomitant use with other hepatotoxic agents (e.g., certain antimicrobials, anticonvulsants, statins) may increase the risk of liver damage, especially with 17α-alkylated androgens.
• Drugs Affecting Testosterone Metabolism:
  • Inducers of CYP3A4 (e.g., rifampin, phenytoin, carbamazepine, St. John’s wort) may increase the metabolic clearance of testosterone, potentially reducing its therapeutic efficacy.
  • Inhibitors of CYP3A4 (e.g., ketoconazole, itraconazole, clarithromycin, ritonavir) may decrease testosterone metabolism, potentially increasing its plasma levels and risk of toxicity.
• Oxyphenbutazone: May increase serum levels of free testosterone by displacing it from SHBG.

Contraindications

Absolute contraindications to testosterone therapy include:

• Known or suspected carcinoma of the prostate or breast in males.
• Pregnancy or breastfeeding, due to risk of fetal virilization.
• Severe cardiac, hepatic, or renal impairment (relative contraindication, requires extreme caution).
• Hypercalcemia associated with metastatic bone disease.
• Known hypersensitivity to testosterone or any component of its formulation.

8. Special Considerations

The use of testosterone in specific populations requires tailored risk-benefit analysis and vigilant monitoring.

Pregnancy and Lactation

Testosterone is contraindicated in pregnancy (FDA Pregnancy Category X). Exposure can cause virilization of the female fetus, leading to clitoromegaly, labial fusion, and ambiguous genitalia. It is also contraindicated during breastfeeding, as androgens can be excreted in milk and cause virilization in the nursing infant.

Pediatric Considerations

Use is restricted to clearly defined medical indications, such as delayed puberty. Dosing must be carefully titrated to avoid premature epiphyseal closure, which results in compromised adult height. Bone age should be monitored regularly via radiography. The potential for abuse among adolescents for performance enhancement is a significant concern.

Geriatric Considerations

Older men may be more susceptible to adverse effects, particularly prostatic hyperplasia (worsening urinary obstruction), polycythemia, and cardiovascular events. A thorough baseline assessment, including digital rectal exam (DRE) and prostate-specific antigen (PSA) measurement, is mandatory. Therapy should be initiated only with clear clinical and biochemical evidence of hypogonadism, and hematocrit, lipid profile, and cardiovascular status must be monitored closely.

Renal and Hepatic Impairment

• Renal Impairment: Caution is advised due to potential fluid retention and increased risk of edema. Dose adjustment may not be strictly necessary for mild to moderate impairment, but monitoring is crucial. Testosterone may be used to treat associated anemia.
• Hepatic Impairment: Use is generally contraindicated in severe liver disease. In mild to moderate impairment, non-alkylated testosterone (e.g., transdermal, injectable esters) may be preferred over 17α-alkylated oral agents due to their lower hepatotoxicity potential. Liver function tests (LFTs) require regular monitoring during therapy.

9. Summary/Key Points

• Testosterone is the primary endogenous androgen, and its pharmacology underpins both replacement therapy and the misuse of anabolic-androgenic steroids.
• Its mechanism of action is predominantly genomic, mediated via the androgen receptor after intracellular activation, often to the more potent metabolite dihydrotestosterone (DHT). Non-genomic signaling pathways also contribute to some rapid effects.
• Pharmacokinetics are formulation-dependent. Intramuscular esters provide depot effects, transdermal systems aim for physiological delivery, and 17α-alkylated oral compounds offer convenience but carry greater hepatotoxic risk.
• The primary therapeutic indication is replacement therapy for documented male hypogonadism. Off-label uses, such as for wasting syndromes or gender affirmation, require careful consideration of risks.
• Adverse effects are systemic and include androgenic changes, HPG axis suppression, polycythemia, potential cardiovascular risks, and (with oral alkylated agents) hepatotoxicity. Boxed warnings address cardiovascular risk, abuse potential, and transdermal gel exposure.
• Significant drug interactions exist with anticoagulants, CYP3A4 modulators, and hypoglycemic agents. It is contraindicated in prostate/breast cancer and pregnancy.
• Special populations require tailored approaches: caution in geriatric patients due to prostate and cardiovascular risks, strict supervision in pediatrics to avoid growth stunting, and avoidance in hepatic impairment.

Clinical Pearls

• The diagnosis of hypogonadism requiring therapy should be based on consistent clinical symptoms and unequivocally low morning serum testosterone levels on at least two occasions.
• When selecting a formulation, consider patient preference, cost, pharmacokinetic profile, and specific risks (e.g., avoid oral alkylated agents in patients with any liver disease).
• Monitoring during therapy is essential and should include: serum testosterone levels (aiming for mid-normal range), hematocrit, PSA, lipid profile, liver function tests (especially with oral agents), and assessment of symptom response and adverse effects.
• Patient education is critical, particularly regarding the realistic expectations of therapy, the risks of abuse, and (for transdermal gels) the prevention of secondary exposure.
• The benefits and risks of testosterone therapy in older men with age-related decline remain a nuanced and evolving area of clinical practice, demanding individualized decision-making.

References

1. Whalen K, Finkel R, Panavelil TA. Lippincott Illustrated Reviews: Pharmacology. 7th ed. Philadelphia: Wolters Kluwer; 2019.
2. Rang HP, Ritter JM, Flower RJ, Henderson G. Rang & Dale's Pharmacology. 9th ed. Edinburgh: Elsevier; 2020.
3. Katzung BG, Vanderah TW. Basic & Clinical Pharmacology. 15th ed. New York: McGraw-Hill Education; 2021.
4. Trevor AJ, Katzung BG, Kruidering-Hall M. Katzung & Trevor's Pharmacology: Examination & Board Review. 13th ed. New York: McGraw-Hill Education; 2022.
5. Golan DE, Armstrong EJ, Armstrong AW. Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy. 4th ed. Philadelphia: Wolters Kluwer; 2017.
6. Brunton LL, Hilal-Dandan R, Knollmann BC. Goodman & Gilman's The Pharmacological Basis of Therapeutics. 14th ed. New York: McGraw-Hill Education; 2023.
7. Rang HP, Ritter JM, Flower RJ, Henderson G. Rang & Dale's Pharmacology. 9th ed. Edinburgh: Elsevier; 2020.
8. Whalen K, Finkel R, Panavelil TA. Lippincott Illustrated Reviews: Pharmacology. 7th ed. Philadelphia: Wolters Kluwer; 2019.