A transgender man on intramuscular testosterone therapy presents with polycythemia. Which pharmacokinetic property of IM testosterone is most responsible for this adverse effect?

• A Intramuscular administration bypasses hepatic first-pass metabolism, increasing bio-availability of testosterone 10-fold compared to oral formulations
• B Testosterone undergoes aromatization to estradiol in muscle tissue, which directly stimulates erythropoiesis via estrogen receptors in bone marrow
• C IM testosterone cypionate or enanthate creates supraphysiological peak serum levels immediately post-injection due to depot release kinetics, stimulating erythropoietin production in a pulsatile manner exceeding physiological androgen stimulation of the kidney ✓
• D Testosterone is converted to DHT (5-alpha reductase) at the injection site, and DHT is more potent than testosterone at stimulating renal EPO production

Explanation

IM testosterone esters (cypionate T1/2 ~8 days, enanthate T1/2 ~4.5 days) create a high-amplitude pulsatile pattern: supraphysiological testosterone peaks (often 1200–1500 ng/dL) in the first 2–3 days post-injection, falling to sub-physiological troughs before the next dose. These supraphysiological peak levels stimulate renal and hepatic EPO production significantly more than physiological testosterone concentrations, leading to polycythemia (hematocrit >54%). This is less pronounced with transdermal testosterone gel/patches or subcutaneous testosterone pellets, which maintain more stable physiological levels. Management involves extending injection intervals, dose reduction, switching delivery method, or therapeutic phlebotomy. Regular hematocrit monitoring (target <54% in most guidelines) is mandated for patients on IM testosterone therapy.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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