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

Effect of Finasteride on Testosterone Levels

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

Mechanism of Action and Hormonal Effects

Finasteride is a competitive and specific inhibitor of Type II 5α-reductase enzyme, which blocks the conversion of testosterone to dihydrotestosterone (DHT) 1. This mechanism has several important hormonal effects:

• Reduces serum DHT concentration by approximately 70% with daily dosing of 5 mg/day for up to 4 years 1
• Increases median circulating testosterone levels by approximately 10-20%, while remaining within the physiologic range 1
• Increases luteinizing hormone (LH) and follicle-stimulating hormone (FSH) by about 10%, but levels remain within normal range 1

The increase in testosterone occurs because finasteride blocks the conversion of testosterone to DHT, resulting in more circulating testosterone. This is a compensatory response to the reduction in DHT levels.

Clinical Implications of Testosterone Changes

The modest increase in testosterone with finasteride has several important clinical implications:

• Prostate Cancer Risk: Finasteride has been shown to reduce the period prevalence of prostate cancer by approximately 26% 2, despite the slight increase in testosterone
• Sexual Function: Sexual side effects associated with finasteride are not due to decreased testosterone but rather to other mechanisms involving DHT reduction
• PSA Levels: Finasteride significantly reduces PSA levels, which requires adjustment when interpreting PSA results for prostate cancer screening 3

Monitoring Considerations

When prescribing finasteride, clinicians should be aware of:

1. PSA Monitoring: For men over 50, consider baseline and periodic PSA testing, applying an approximately 2x multiplier to interpret PSA results when on finasteride 2
2. Sexual Function: Regular assessment of sexual function is recommended during treatment, as finasteride is associated with sexual side effects including erectile dysfunction, decreased libido, and ejaculatory disorders 2
3. Hormone Levels: Routine monitoring of testosterone levels is not necessary as the changes remain within physiologic range 1

Potential Side Effects

While finasteride does not decrease testosterone, it is associated with several side effects:

• Sexual dysfunction (erectile dysfunction, decreased libido, ejaculatory disorders) 2, 4
• Gynecomastia (breast enlargement) in approximately 2.2% of patients 2
• Breast tenderness in 0.7% of patients 2

Clinical Recommendation

For patients requiring finasteride therapy (for BPH or androgenetic alopecia), clinicians should:

• Counsel patients that finasteride does not decrease testosterone levels
• Inform patients about potential sexual side effects, which are related to DHT reduction rather than testosterone changes
• Consider the slight increase in testosterone when interpreting other laboratory values or symptoms

The slight increase in testosterone with finasteride therapy is not clinically concerning and should not deter appropriate use of the medication in indicated patients.