Does finasteride (5-alpha-reductase inhibitor) cause hypogonadism (low testosterone)?

Finasteride and Testosterone Levels

Finasteride does not cause low testosterone; in fact, it typically increases serum testosterone levels by approximately 10-20% while remaining within the physiologic range. 1

Mechanism of Action and Hormonal Effects

• Finasteride is a competitive and specific inhibitor of Type II 5α-reductase enzyme, which converts testosterone to dihydrotestosterone (DHT) 1
• Finasteride forms a stable enzyme complex with 5α-reductase, effectively blocking the conversion of testosterone to DHT 1, 2
• Daily dosing of finasteride at 5 mg/day reduces serum DHT concentration by approximately 70% 1
• The median circulating level of testosterone increases by approximately 10-20% but remains within the physiologic range 1, 3
• In a separate study of men treated with finasteride 1 mg per day, mean circulating levels of testosterone and estradiol increased by approximately 15% compared to baseline, while still remaining within normal ranges 1

Endocrine System Effects

• Finasteride treatment leads to small increases (about 10%) in luteinizing hormone (LH) and follicle-stimulating hormone (FSH), but these levels remain within normal ranges 1
• The hypothalamic-pituitary-testicular axis is not significantly affected by finasteride, as evidenced by unaltered response of LH and FSH to gonadotropin-releasing hormone 1
• Finasteride has no effect on circulating levels of cortisol, prolactin, thyroid-stimulating hormone, or thyroxine in patients with BPH 1
• No clinically meaningful effect has been observed on plasma lipid profiles or bone mineral density 1

Clinical Considerations and Side Effects

• The primary hormonal effect of finasteride is DHT suppression, not testosterone reduction 3, 2
• In prostatic tissue, DHT content decreases by approximately 80% after finasteride treatment, while testosterone tissue concentration increases up to 10 times over pretreatment levels 1
• Some studies report that continued administration of finasteride 5 mg/day can alter serum levels of testosterone, DHT, FSH, and LH significantly 4
• Sexual side effects (erectile dysfunction, decreased libido, ejaculation disorders) are reported in approximately 2-4% more patients taking finasteride compared to placebo 5, 6
• 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) 5, 6

Important Clinical Pitfalls

• Discontinuation of finasteride therapy results in a return of DHT levels to pretreatment levels in approximately 2 weeks 1
• When monitoring PSA levels in patients taking finasteride, clinicians should be aware that PSA is suppressed by approximately 50% after 12 months of therapy 5
• Sexual dysfunction should be monitored in patients taking finasteride, as it can affect quality of life in a substantial number of patients 4
• The magnitude of sexual dysfunction effect is relatively small compared to natural aging (finasteride effect: 3.21 points; each year of aging: 1.26 points on sexual function scale) 5, 6

In conclusion, finasteride does not cause hypogonadism or low testosterone levels. Instead, it typically causes a modest increase in testosterone levels while significantly decreasing DHT levels through 5α-reductase inhibition.