Finasteride Does Not Lower Testosterone Levels—It Increases Them
Finasteride does not lower testosterone; instead, it increases serum testosterone levels by approximately 10-20% while dramatically reducing dihydrotestosterone (DHT) by approximately 70%, with both hormones remaining within physiologic ranges. 1
Mechanism and Hormonal Effects
Finasteride works by inhibiting the Type II 5α-reductase enzyme, which converts testosterone to DHT. 1 This blockade creates a predictable hormonal shift:
- Testosterone increases by 10-20% above baseline in men taking finasteride 5 mg daily, and by approximately 15% in men taking 1 mg daily 1
- DHT decreases by approximately 70% in serum with the 5 mg dose 1
- Both hormones remain within normal physiologic ranges throughout treatment 1
The FDA label explicitly states that finasteride produces "a rapid reduction in serum DHT concentration" while "the median circulating level of testosterone increased by approximately 10 to 20% but remained within the physiologic range." 1
Additional Endocrine Effects
Finasteride's hormonal impact extends beyond testosterone and DHT:
- Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) increase by approximately 10% but remain within normal ranges 1
- Estradiol levels increase by approximately 15% compared to baseline, remaining within physiologic range 1
- The hypothalamic-pituitary-testicular axis remains intact, as demonstrated by preserved LH and FSH response to gonadotropin-releasing hormone 1
- No effect on cortisol, prolactin, thyroid-stimulating hormone, or thyroxine 1
Clinical Implications
The American Urological Association guidelines confirm that finasteride "lowers prostatic dihydrotestosterone, but not to castration levels" and emphasize that the drug does not produce hypogonadism. 2
Age-Related Considerations
Research demonstrates that younger men (≤26 years) have higher baseline DHT levels and experience a more pronounced 50% decrease in DHT by 12 months of treatment with finasteride 1 mg, which correlates with better hair growth response. 3 This age group shows the same testosterone-increasing effect as older men.
Metabolic Profile Changes
In men treated with finasteride 1 mg for androgenetic alopecia, the expected steroid spectrum changes include decreased DHT alongside increased testosterone, androstenedione, and free testosterone index. 4 These changes may produce mild improvements in glucose metabolism regulation, with decreased glycated hemoglobin (HbA1c) observed after 12 months. 4
Common Misconception
The sexual side effects associated with finasteride (occurring in 2-4% more patients than placebo) are not due to testosterone deficiency. 2, 5 Rather, they result from DHT reduction in specific tissues. The magnitude of sexual dysfunction is relatively small—a mean difference of 3.21 points on sexual function scales compared to 1.26 points for each year of aging. 5
Important Caveat
While finasteride increases circulating testosterone, it reduces DHT specifically in target tissues like the prostate and scalp. 1 The drug has no affinity for the androgen receptor itself and works purely through enzyme inhibition. 1