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

Effect of Finasteride on Testosterone Levels

Finasteride does not decrease testosterone levels; in fact, it causes a mild increase in serum testosterone levels by approximately 10-20% while maintaining levels within the physiologic range. 1

Mechanism of Action and Hormonal Effects

• Finasteride is a competitive and specific inhibitor of Type II 5α-reductase enzyme that blocks the conversion of testosterone to dihydrotestosterone (DHT) 1
• While finasteride significantly reduces DHT levels (by approximately 70% with 5mg daily dosing), it actually increases circulating testosterone levels by about 10-20% 1
• This increase in testosterone occurs because the testosterone that would normally be converted to DHT remains as testosterone when 5α-reductase is inhibited 1
• In patients with BPH treated with finasteride, testosterone tissue concentration in the prostate was increased up to 10 times over pretreatment levels compared to placebo 1

Dosage Considerations

• At therapeutic doses (5mg/day for BPH or 1mg/day for male pattern hair loss), finasteride selectively inhibits the type 2 isoform of 5α-reductase 1
• Even at the lower 1mg daily dose used for male pattern baldness, finasteride has been shown to have similar effects on hormone levels as the 5mg dose in men aged 50 years and older 2
• Daily dosing with finasteride 5mg for up to 4 years maintains the suppression of DHT while keeping testosterone levels elevated but within normal range 1

Other Hormonal Effects

• In addition to the mild increase in testosterone, finasteride treatment is associated with approximately 15% increases in luteinizing hormone (LH) and follicle-stimulating hormone (FSH), though these also remain within normal ranges 1
• Finasteride has no effect on circulating levels of cortisol, prolactin, thyroid-stimulating hormone, or thyroxine 1
• No clinically meaningful effect has been observed on plasma lipid profiles or bone mineral density 1

Clinical Implications

• The increase in testosterone does not typically lead to androgenic side effects, as DHT (not testosterone) is the primary androgen responsible for conditions like BPH and male pattern baldness 3
• Despite the increase in testosterone, sexual side effects are still reported, including erectile dysfunction, decreased libido, and ejaculation disorders in approximately 2-4% more patients taking finasteride compared to placebo 4
• In the Prostate Cancer Prevention Trial (PCPT), 67.4% of men on finasteride reported erectile dysfunction compared to 61.5% on placebo (relative risk 1.10) 2
• The magnitude of sexual dysfunction effect is relatively small compared to natural aging, with finasteride contributing 3.21 points and each year of aging contributing 1.26 points on the sexual function scale 4

Common Pitfalls and Monitoring

• When monitoring PSA levels in patients taking finasteride, be aware that PSA is suppressed by approximately 50% after 12 months of therapy 4
• The effects of finasteride on testosterone and DHT are reversible; DHT levels return to pretreatment levels approximately 2 weeks after discontinuation 1
• Clinicians should not attribute symptoms of hypogonadism to finasteride therapy, as the medication actually increases rather than decreases testosterone levels 5

In summary, finasteride treatment results in a mild increase in serum testosterone levels while significantly reducing DHT levels. This hormonal profile is maintained throughout long-term treatment, with testosterone levels remaining within the physiologic range.