The 5-Alpha-Reductase Enzyme: Type 1 vs. Type 2 and Why Finasteride Picks One

Finasteride is a selective 5-alpha-reductase inhibitor. Understanding what that means — which enzyme, which tissues, and what the DHT produced there does — makes the medication's clinical behaviour legible.

5-alpha-reductase (5-AR) is the enzyme that converts testosterone to dihydrotestosterone (DHT). It is not one enzyme but a family of enzymes — specifically, two distinct isoforms with different tissue distributions, different kinetics, and different susceptibility to inhibitors. Understanding which isoform does what, and which one finasteride targets, makes the medication's behaviour — including its efficacy ceiling and why dutasteride is different — mechanistically clear.

What 5-alpha-reductase does

The enzyme catalyses the reduction of testosterone at the 4,5 double bond, producing 5-alpha-dihydrotestosterone. DHT is a potent androgen — it binds to androgen receptors with approximately five times greater affinity than testosterone and dissociates more slowly, producing a more prolonged receptor activation.

In tissues where androgen activity drives function — the prostate, hair follicles in susceptible individuals, sebaceous glands — the local availability of DHT matters more than systemic testosterone levels. This is why some men with normal testosterone levels still experience significant androgenetic alopecia: the conversion happening in the follicle produces local DHT concentrations that drive miniaturisation regardless of what is happening systemically.

Type 1 and Type 2: the tissue distribution

The two clinically relevant isoforms have distinct but overlapping distributions.

Type 1 (SRD5A1)

Type 1 is expressed most prominently in the liver, skin, and sebaceous glands. It is the predominant isoform in non-balding scalp skin. It also has activity in the adrenal glands and contributes to systemic DHT production, particularly in peripheral tissues. Type 1 has relatively higher kinetics (faster enzyme turnover) and different substrate affinities than type 2.

Type 2 (SRD5A2)

Type 2 is expressed most prominently in the prostate, seminal vesicles, epididymis, and — critically for androgenetic alopecia — in the dermal papilla cells of the hair follicle. It is the predominant isoform responsible for DHT production within the follicle itself, where that DHT directly signals the androgen receptor and drives miniaturisation. Type 2 has slower kinetics but higher affinity for testosterone as a substrate.

Why finasteride is selective for type 2

Finasteride was developed with the prostate as the primary target — the original clinical indication is benign prostatic hyperplasia (BPH), where type 2 dominates. The drug's structure makes it a competitive inhibitor of type 2 with approximately 100-fold greater selectivity for type 2 over type 1.

For androgenetic alopecia, this selectivity is appropriate: the dermal papilla cells that drive miniaturisation primarily express type 2. Finasteride 1mg daily reduces serum DHT by approximately 70% and scalp DHT by approximately 60–70%. The reduction in scalp DHT reduces the signal driving follicle miniaturisation.

What finasteride does not address: the type 1 activity in the liver and peripheral skin that contributes to systemic DHT. This is why finasteride does not eliminate DHT entirely and why there is a suppression ceiling.

The dutasteride comparison

Dutasteride inhibits both type 1 and type 2 isoforms. This produces DHT suppression of approximately 90–95% compared to finasteride's 65–70% in serum. For hair loss, dutasteride typically reduces scalp DHT by approximately 51% beyond what finasteride achieves. In head-to-head studies, dutasteride 0.5mg outperforms finasteride 1mg on hair count and density measures.

The trade-off: deeper DHT suppression affects more tissues. The longer half-life (five weeks for dutasteride versus six hours for finasteride) means that recovery of DHT levels after discontinuation takes considerably longer — relevant for men considering a pause for fertility reasons.

For the majority of people starting a 5-AR inhibitor for the first time, finasteride is the standard first-line choice: well-studied, available generically at low cost, and effective for most responders. Dutasteride is a logical upgrade for documented finasteride non-responders or for clinicians seeking deeper suppression from the start.

What this means for your prescription

Understanding the enzyme selectivity explains the mechanism; it does not change the practical prescription process. What it does do: it gives you a framework for interpreting response (why some people respond fully, some partially, some minimally), for understanding the transition to dutasteride if finasteride's ceiling is reached, and for understanding why topical formulations targeting the scalp directly can be complementary rather than redundant.