In an adult male taking standard doses of finasteride (1 mg daily for androgenic alopecia or 5 mg daily for benign prostatic hyperplasia), does finasteride affect serum testosterone levels?

Finasteride Increases Serum Testosterone by 10-20%

Yes, finasteride does affect testosterone levels—it increases serum testosterone by approximately 10-20% while dramatically reducing DHT by about 70%, but these testosterone levels remain within the normal physiologic range. 1, 2

Mechanism of Action

Finasteride works by competitively inhibiting the 5α-reductase type II enzyme, which normally converts testosterone to dihydrotestosterone (DHT). 2 When this conversion is blocked:

• Serum DHT decreases by approximately 70% 1, 2
• Prostatic DHT decreases by 85-90% 3, 4
• Serum testosterone increases by 10-20% as a compensatory response because less testosterone is being converted to DHT 1, 2
• The increase in testosterone is greatest in men with low baseline testosterone levels, with one study showing associated reductions in body mass index of 0.6-0.8 kg/m² in these patients 5

Timeline and Stability of Hormonal Changes

• Maximum DHT suppression occurs 8 hours after the first dose 2
• The testosterone increase of approximately 15% is observed with the 1 mg daily dose (used for androgenic alopecia) 2
• Both 1 mg and 5 mg doses produce similar hormonal effects in men aged 50 years and older 1
• These hormonal changes persist throughout the 24-hour dosing interval with continued daily treatment 2

Clinical Significance: Why This Matters

The modest increase in testosterone does NOT prevent sexual side effects. 1 This is a critical point:

• Sexual dysfunction (decreased libido, erectile dysfunction, ejaculatory dysfunction) still occurs in approximately 2-4% more patients taking finasteride compared to placebo 1
• The Prostate Cancer Prevention Trial found 67.4% of men on finasteride reported erectile dysfunction versus 61.5% on placebo (relative risk 1.10) 1
• These sexual side effects are reversible and uncommon after the first year of therapy 6

Hypothalamic-Pituitary-Testicular Axis Remains Intact

Finasteride does not disrupt the hormonal feedback loop. 2 Important findings include:

• Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) increase by about 10% but remain within normal range 2
• The hypothalamic-pituitary-testicular axis response to gonadotropin-releasing hormone remains normal 2
• No clinically meaningful effects on cortisol, prolactin, thyroid-stimulating hormone, or thyroxine 2

Common Pitfalls to Avoid

Do not assume the testosterone increase will improve sexual function or fertility. 1, 7 The elevation in testosterone:

• Works by peripheral enzyme inhibition, not by stimulating testicular production 1
• Does not aid in fertility restoration protocols (such as post-cycle therapy) because it doesn't reactivate the hypothalamic-pituitary-gonadal axis 7
• The 5 mg dose is associated with reduced semen volume, though evidence for the 1 mg dose on fertility is inconclusive 7

Monitor for gynecomastia, which occurs in 4.5% of finasteride users versus 2.8% on placebo (relative risk ≈ 1.64), likely related to the altered testosterone-to-estrogen ratio. 1