Does Finasteride Affect Testosterone Levels?
Yes, finasteride increases serum testosterone levels by approximately 10-20% while dramatically reducing dihydrotestosterone (DHT) by about 70%, though testosterone remains within the normal physiologic range. 1, 2
Mechanism of Action
Finasteride works as a competitive inhibitor of the 5α-reductase type II enzyme, which normally converts testosterone to the more potent androgen DHT. 1 By blocking this conversion, the drug causes:
- Serum testosterone increase of 10-20% above baseline, remaining within normal physiologic limits 1, 2
- Serum DHT reduction of approximately 70% in circulation 1
- Prostatic DHT reduction to 15% or less of baseline levels after just 7 days of treatment 3
- Reciprocal increase in prostatic testosterone up to 10-fold over pretreatment levels 2, 3
Clinical Context and Hormonal Changes
The FDA label confirms that in men receiving finasteride 5 mg daily, the median circulating testosterone level increased by 10-20% but remained within physiologic range. 2 Similarly, in studies using finasteride 1 mg daily for androgenetic alopecia, mean testosterone and estradiol levels increased by approximately 15% compared to baseline. 2
Compensatory pituitary response: Finasteride causes increases of about 10% in luteinizing hormone (LH) and follicle-stimulating hormone (FSH), though these remain within normal range, indicating the hypothalamic-pituitary-testicular axis remains functional. 2
Important Clinical Caveats
Despite the increase in testosterone, sexual side effects still occur. 1 The European Association of Urology notes that sexual dysfunction occurs in approximately 2-4% more patients taking finasteride compared to placebo. 1 The Prostate Cancer Prevention Trial found erectile dysfunction in 67.4% of men on finasteride versus 61.5% on placebo (relative risk 1.10). 1
This paradox occurs because DHT, not testosterone, is the primary androgen in the prostate and hair follicles, and finasteride's dramatic reduction in DHT outweighs the modest testosterone increase for these tissues. 4, 5
No effect on other hormones: Finasteride does not alter circulating levels of cortisol, prolactin, thyroid-stimulating hormone, thyroxine, or plasma lipid profiles. 2
Monitoring Implications
When following patients on finasteride, remember that PSA levels are suppressed by approximately 50% after 12 months of therapy due to prostatic DHT reduction and subsequent prostate volume decrease. 1 This requires doubling the PSA value when screening for prostate cancer in men on finasteride therapy. 6