Does finasteride (5-alpha-reductase inhibitor) lower testosterone levels?

Effects of Finasteride on Testosterone Levels

Finasteride does not lower testosterone levels; in fact, it typically causes a mild increase in serum testosterone by approximately 10-20% while remaining within the physiologic range.

Mechanism of Action and Hormonal Effects

Finasteride works by inhibiting the 5α-reductase enzyme, which is responsible for converting testosterone to dihydrotestosterone (DHT). This mechanism has several important hormonal effects:

• Finasteride significantly reduces DHT levels (by approximately 70% with the 5mg daily dose) 1
• Serum testosterone levels typically increase by 10-20%, while remaining within normal physiologic range 1
• Estradiol levels may also increase by approximately 15% 1
• Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) may increase by about 10%, but generally remain within normal ranges 1

The FDA drug label clearly states: "In patients receiving finasteride tablets 5 mg/day, increases of about 10% were observed in luteinizing hormone (LH) and follicle-stimulating hormone (FSH), but levels remained within the normal range." 1

Clinical Significance

The hormonal changes associated with finasteride have several important clinical implications:

• Prostate Effects: Finasteride reduces prostate volume by approximately 20% through DHT suppression 2
• PSA Effects: Finasteride lowers serum PSA levels, which is important to consider when screening for prostate cancer 2
• Sexual Function: The slight increase in testosterone does not prevent sexual side effects, which can include decreased libido, ejaculatory dysfunction, and erectile dysfunction 2, 3

Differences Between Dosages

• The 5mg dose (used for BPH) and 1mg dose (used for androgenetic alopecia) have similar effects on the hormonal profile
• Both doses inhibit 5α-reductase, though the 5mg dose may have more pronounced effects 4
• The 1mg dose has been specifically optimized for hair loss treatment while minimizing side effects 5

Common Pitfalls and Misconceptions

1. Confusing testosterone with DHT: Many patients and some clinicians confuse the effects of finasteride on testosterone versus DHT. Finasteride significantly reduces DHT but slightly increases testosterone.

2. Overlooking PSA implications: When monitoring PSA in men on finasteride, values should be approximately doubled to maintain comparable sensitivity for prostate cancer detection 2

3. Assuming hormonal normalization is immediate after discontinuation: After stopping finasteride, DHT levels typically return to pretreatment levels in approximately 2 weeks 1

Clinical Recommendations

For patients concerned about hormonal effects when considering finasteride:

• Reassure them that testosterone levels will not be reduced
• Explain that the slight increase in testosterone is not clinically significant for muscle mass, energy levels, or overall vigor
• Discuss the potential sexual side effects despite the maintenance of normal testosterone levels
• Consider baseline hormonal assessment in men with pre-existing hormonal concerns

In conclusion, while finasteride significantly reduces DHT levels, it does not lower testosterone and may actually cause a slight increase while remaining within the physiologic range.