A 26-year-old amateur bodybuilder self-administers high-dose testosterone injections. He now presents with bilateral gynaecomastia. The mechanism responsible is:

• A Direct binding of testosterone to breast estrogen receptors
• B Conversion of testosterone to dihydrotestosterone in breast tissue
• C Testosterone induced destruction of Leydig cells raising LH
• D Peripheral aromatisation of supraphysiological testosterone to estradiol ✓

Explanation

Exogenous testosterone is aromatised peripherally in adipose tissue and breast tissue by the enzyme aromatase (CYP19) to estradiol. At supraphysiological doses this estradiol load stimulates ductal proliferation, producing gynaecomastia despite the androgenic milieu. Dihydrotestosterone formation via 5-alpha reductase drives prostate and skin effects, not breast growth, so option B fails. This same aromatisation explains why anastrozole is sometimes added illicitly to steroid regimens.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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