Blocks both forms of the enzyme that converts testosterone into the hormone driving inherited pattern hair loss; the older licensed alternative blocks only one. Short trials show more and thicker hair than that older medication. Use is indefinite. Nearly all evidence is manufacturer-funded, stops at six months, and enrolled only men. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Prostate-specific antigen | Below 1.0 ng/mL under age 60; thereafter track rises from the on-treatment low point | Detects prostate cancer despite drug-induced suppression |
| Total testosterone | 600–900 ng/dL | Rises as conversion to dihydrotestosterone is blocked; confirms the expected hormonal shift |
| Dihydrotestosterone | No established on-treatment target; track the fall from the pre-treatment value, expected above 90% | Confirms absorption and adequate enzyme blockade |
| Oestradiol | 20–30 pg/mL | Breast tenderness and gynaecomastia track a rising oestrogen-to-androgen ratio |
| Ferritin | 70–100 ng/mL | Low iron stores cause diffuse shedding that no androgen blocker will correct |
| 25-hydroxyvitamin D | 40–60 ng/mL | Deficiency is associated with disordered hair cycling |
| Thyroid-stimulating hormone | 0.5–2.0 mIU/L | Thyroid dysfunction produces shedding that mimics treatment failure |
| Alanine aminotransferase | Below 25 U/L in men, below 20 U/L in women | Clearance is hepatic, so impairment raises drug exposure |
| Semen analysis | At least 39 million total sperm and 40% motility | Establishes a pre-treatment baseline where fatherhood is planned |
Cadence: Baseline before the first capsule; hormones and prostate-specific antigen repeated at 6 months, then every 12 months; photography at 6 and 12 months, then annually