Does Finasteride Affect Testosterone Levels?
Finasteride does not significantly decrease testosterone levels; in fact, it causes a modest increase in circulating testosterone of approximately 10-20% while profoundly suppressing dihydrotestosterone (DHT) by approximately 70%. 1
Mechanism and Hormonal Effects
Finasteride works by competitively inhibiting Type II 5α-reductase, the enzyme that converts testosterone to DHT in the prostate, liver, and skin. 1 This selective inhibition creates a predictable hormonal profile:
Testosterone Changes
- Circulating testosterone increases by 10-20% in men taking finasteride 5 mg daily, remaining within the physiologic range throughout treatment. 1
- In men taking finasteride 1 mg daily, testosterone levels increase by approximately 15% compared to baseline, but stay within normal physiologic limits. 1
- These testosterone elevations are maintained chronically during treatment and do not require dose adjustment. 2
DHT Suppression
- Serum DHT concentration decreases by approximately 70% with daily dosing of finasteride 5 mg. 1
- Maximum DHT suppression occurs 8 hours after the first dose and is maintained throughout the 24-hour dosing interval. 1
- Prostatic tissue DHT declines to 15% or less of baseline levels after just 7 days of treatment at any dose (1-100 mg/day). 3
Other Hormonal Parameters
- Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) increase by approximately 10% but remain within normal range. 1
- The hypothalamic-pituitary-testicular axis remains functionally intact, as demonstrated by preserved LH and FSH response to gonadotropin-releasing hormone. 1
- Estradiol levels increase by approximately 15% in men taking 1 mg daily, remaining within physiologic range. 1
- No clinically meaningful effects occur on cortisol, prolactin, thyroid-stimulating hormone, thyroxine, lipid profiles, or bone mineral density. 1
Clinical Implications
Sexual Function Preservation
The increase in testosterone (rather than decrease) is clinically relevant because testosterone, not DHT, is the primary androgen responsible for libido, erectile function, and muscle strength through its humoral endocrine and paracrine effects. 4 This explains why:
- Sexual dysfunction rates with finasteride are modest (2-4% above placebo for erectile dysfunction and decreased libido). 5
- The magnitude of sexual dysfunction is small: only 3.21 points on a 0-100 scale compared to 1.26 points for each year of aging. 5
- Muscle strength and bone density are preserved since testosterone mediates these effects. 6, 2
Reversibility
- Upon discontinuation, DHT levels return to pretreatment values in approximately 2 weeks. 1
- Prostate volume returns to near-baseline approximately 3 months after stopping therapy. 1
Important Caveats
Do not confuse testosterone levels with DHT levels—finasteride profoundly suppresses DHT while modestly increasing testosterone. 1 This distinction is critical for counseling patients who may mistakenly believe finasteride causes "low testosterone."
The testosterone increase is a compensatory physiologic response to DHT suppression and does not indicate hyperandrogenism or require monitoring beyond routine clinical assessment. 2