From the Guidelines
Testosterone levels in a patient taking finasteride should be interpreted with caution, as finasteride does not significantly alter total testosterone levels but may affect other hormonal parameters. When reviewing testosterone results in these patients, it's essential to understand that finasteride works by inhibiting the enzyme 5-alpha reductase, which converts testosterone to dihydrotestosterone (DHT) 1. This typically results in decreased DHT levels (by approximately 70%) while total testosterone levels remain relatively unchanged or may slightly increase. If the patient's testosterone levels are abnormal, this is likely not due to finasteride and warrants further investigation. Additionally, consider checking DHT levels, as this is the hormone directly affected by finasteride. Free testosterone may also be worth measuring, as it represents the biologically active form. When discussing results with the patient, explain that any sexual side effects experienced while on finasteride are typically related to DHT reduction rather than changes in testosterone levels. The timing of the test is not particularly important in relation to finasteride dosing, as the drug's effect on the 5-alpha reductase enzyme is relatively constant throughout the treatment period 1.
Some key points to consider when interpreting testosterone levels in patients taking finasteride include:
- Finasteride's effect on 5-alpha reductase and the subsequent decrease in DHT levels
- The potential for slight increases in total testosterone levels
- The importance of measuring free testosterone and DHT levels for a comprehensive understanding of the patient's hormonal profile
- The need to investigate abnormal testosterone levels further, as they are unlikely to be caused by finasteride
- The relationship between DHT reduction and sexual side effects in patients taking finasteride.
It's crucial to prioritize patient-centered outcomes and consider the potential benefits and harms of testosterone treatment in men, as highlighted in the evidence reports from the American College of Physicians 1.
From the FDA Drug Label
- 5 Effect on Semen Characteristics Treatment with finasteride tablets for 24 weeks to evaluate semen parameters in healthy male volunteers revealed no clinically meaningful effects on sperm concentration, mobility, morphology, or pH.
The FDA drug label does not answer the question.
From the Research
Interpreting Testosterone Levels in Men Taking Finasteride
When interpreting testosterone levels in men taking finasteride, several factors should be considered:
- Finasteride is a 5α-reductase inhibitor that lowers dihydrotestosterone (DHT) levels, but its effect on testosterone levels is less clear 2, 3, 4.
- Studies have shown that finasteride can increase testosterone levels by 15% after 3 months of treatment 4.
- The increase in testosterone levels is likely due to the inhibition of 5α-reductase, which normally converts testosterone to DHT 3, 4.
- However, the clinical significance of this increase in testosterone levels is unclear, and more research is needed to fully understand its effects 5.
Effects of Finasteride on Hormone Levels
Finasteride has been shown to have the following effects on hormone levels:
- Decrease in DHT levels by 60% after 3 months of treatment 4.
- Increase in testosterone levels by 15% after 3 months of treatment 4.
- Decrease in follicle-stimulating hormone (FSH) and luteinizing hormone (LH) levels by 24% and 16%, respectively 4.
- No significant changes in prolactin, aldosterone, cortisol, and dehydroepiandrosterone levels 4.
Clinical Implications
The clinical implications of these changes in hormone levels are:
- Finasteride may cause sexual dysfunction in some men, including erectile dysfunction 6, 4.
- Finasteride may improve symptoms of benign prostatic hyperplasia (BPH) and reduce prostate volume 2, 6.
- The long-term effects of finasteride on hormone levels and clinical outcomes are not fully understood and require further study 5.