Finasteride Does Not Lower Testosterone—It Actually Increases It
Finasteride does not lower testosterone levels; instead, it increases serum testosterone by approximately 10-20% while dramatically reducing dihydrotestosterone (DHT) by about 70%. This is the fundamental mechanism of action that clinicians must understand when counseling patients 1.
Mechanism of Action
Finasteride works by inhibiting the 5α-reductase type II enzyme, which normally converts testosterone to the more potent androgen DHT 2. By blocking this conversion:
- Serum DHT decreases by approximately 70% with finasteride 5mg daily 1
- Serum testosterone increases by 10-20% as it is no longer being converted to DHT 1
- Testosterone levels remain within the normal physiologic range despite this increase 1
The median circulating testosterone level increases by approximately 10-20% but stays within normal limits 1. In a study of healthy men treated with finasteride 1mg daily, mean circulating levels of testosterone and estradiol increased by approximately 15% compared to baseline, but remained within the physiologic range 1.
Hormonal Effects Beyond Testosterone
Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) increase by about 10% in patients receiving finasteride 5mg daily, but levels remain within the normal range 1. Importantly, finasteride does not alter the response of LH and FSH to gonadotropin-releasing hormone, indicating that the hypothalamic-pituitary-testicular axis remains intact 1.
Finasteride has no clinically meaningful effect on 1:
- Cortisol levels
- Prolactin levels
- Thyroid-stimulating hormone
- Thyroxine
- Plasma lipid profile (total cholesterol, LDL, HDL, triglycerides)
- Bone mineral density
Tissue-Level Effects
While serum testosterone increases, intraprostatic DHT content decreases by approximately 80% in men treated with finasteride for 7-10 days prior to prostatectomy 1. Paradoxically, testosterone tissue concentration in the prostate increases up to 10 times over pretreatment levels relative to placebo, but this elevated testosterone cannot compensate for the dramatic reduction in DHT 1.
Clinical Implications
Sexual Function Considerations
Despite the increase in serum testosterone, sexual side effects still occur 3. This is a critical point for patient counseling:
- Sexual dysfunction occurs in approximately 2-4% more patients taking finasteride compared to placebo 3
- In the Prostate Cancer Prevention Trial, 67.4% of men on finasteride reported erectile dysfunction compared to 61.5% on placebo (relative risk 1.10) 3
- The magnitude of sexual dysfunction attributable to finasteride is relatively small—finasteride contributes 3.21 points on the sexual function scale, while each year of aging contributes 1.26 points 3
Common Pitfall: Confusing DHT Reduction with Testosterone Reduction
The most common misconception is that because finasteride is an "anti-androgen," it must lower testosterone. This is incorrect. Finasteride is tissue-selective, reducing DHT primarily in the prostate, scalp, and genital skin where 5α-reductase type II is concentrated 2. The systemic increase in testosterone is a compensatory response to blocked DHT production.
Dose Considerations
Both 1mg (for hair loss) and 5mg (for BPH) doses produce similar hormonal effects in men aged 50 years and older 3. The lower 1mg dose still increases testosterone and decreases DHT, though the magnitude of DHT suppression may be slightly less than with 5mg dosing 1.
Monitoring Implications
When monitoring patients on finasteride, remember that PSA levels are suppressed by approximately 50% after 12 months of therapy 3. This is due to the reduction in prostatic DHT and subsequent prostate volume reduction, not due to testosterone changes 2.