What does finasteride actually do to your hormones?
It shifts the balance between testosterone and DHT rather than lowering testosterone. Finasteride blocks type II 5-alpha reductase, the enzyme that converts testosterone into DHT, the more potent hormone that shrinks scalp follicles in pattern hair loss.
The Propecia label gives the numbers:
| Hormone | Change on finasteride 1 mg | What it means |
|---|---|---|
| DHT in the blood | About 65% lower within 24 hours | The intended effect |
| Testosterone | About 15% higher on average | Stays within the normal range |
| Estradiol | About 15% higher on average | Stays within the normal range |
| LH, FSH, prolactin | No clinically meaningful change | The brain’s signal to the testes is unaffected |
So when testosterone is blocked from becoming DHT, a little more of it stays as testosterone. MedlinePlus describes finasteride as a 5-alpha reductase inhibitor that treats pattern hair loss by blocking the body’s production of a male hormone in the scalp, not by switching off testosterone.
If testosterone does not drop, why do some men lose libido?
That is the puzzle at the center of finasteride side effects. In the trials, a small share of men reported lower libido or erection changes, and how finasteride causes them in those men is still not well understood.
A 2016 study of men with sexual symptoms that persisted after stopping finasteride found their testosterone and DHT levels were no different from finasteride users without symptoms or from men who had never taken it. Their depression scores were higher and their brain scans resembled patterns seen in depression. In practice, this means a testosterone test usually will not explain finasteride-related symptoms, though it is still worth checking if symptoms started before the medicine. Our answer on whether finasteride side effects are permanent covers this in more depth.
Why does the distinction matter for men weighing TRT?
Because the two problems need different fixes. If your libido dropped after starting finasteride and your testosterone is normal, testosterone therapy is not the answer. If your symptoms predate finasteride and your testosterone is genuinely low, finasteride is not the cause either.
Getting that right depends on testing well. The Endocrine Society says a diagnosis of low testosterone needs at least two early morning blood tests, between 7 and 10 a.m., showing low levels along with typical signs and symptoms. MedlinePlus explains that levels are usually highest in those morning hours. Our answer on what testosterone level qualifies you for TRT walks through the cutoffs.
Can you take finasteride and TRT together?
Men ask because TRT raises testosterone, and more testosterone means more raw material for DHT. That is why hair often comes up in TRT conversations, and why finasteride sometimes enters the discussion. Our article on low testosterone, TRT and DHT explains that link.
Two practical points apply. First, the clinician managing TRT decides whether finasteride fits your plan. Second, the Propecia label notes that finasteride lowers PSA, by about 50% in older men taking the 5 mg dose for an enlarged prostate, so PSA results during TRT monitoring must be read with that in mind. Any confirmed rise from your lowest PSA on finasteride should be evaluated. See PSA on finasteride for the details.
Related question: does finasteride affect fertility?
It can affect semen in some men. The Propecia label reports a small median drop in ejaculate volume with the 1 mg dose over 48 weeks, and lists postmarketing reports of infertility or poor semen quality, with improvement or normalization reported after stopping. Men planning a family should raise this, especially if TRT is also on the table. The American Society for Reproductive Medicine notes that testosterone treatment usually leads to low or absent sperm counts, which typically recover within about three months of stopping.
How Ultimate Male sorts out hair and hormone questions
At our San Gabriel and Downey clinics, men often arrive with both questions at once: thinning hair and low energy or libido. The provider separates them with a morning lab draw, including the TRT pre-screening panel when testosterone is in question, and a review of when each symptom began relative to any medicine.
From there, hair goes down one path and hormones down another. For hair, that may mean PRP hair restoration with or without a medicine; for confirmed low testosterone, a separate conversation about testosterone therapy. A consultation is an evaluation, so neither treatment is assumed before the results are in.
