Your Hair is Falling Out, but Your Bloodwork Looks Perfect. Here's the Biological Reason.

The androgen receptor is a protein that sits on your hair follicle cells and responds to DHT (dihydrotestosterone), the hormone most directly implicated in hair loss. Think of it as the lock, and DHT as the key. The more sensitive the lock, the more readily DHT triggers hair follicle miniaturization (shrinking). Your AR gene determines this sensitivity through a repeating segment of DNA called a CAG repeat. The length of this repeat directly affects how responsive your follicles are to DHT.

People with shorter CAG repeats have a more sensitive androgen receptor. This means their hair follicles respond more aggressively to DHT, even when DHT levels are normal. Some estimates suggest that roughly 30-50% of the population carries the variant associated with higher androgen receptor sensitivity. Your hair follicles can be hypersensitive to DHT without you having abnormally high DHT levels, and standard hormone testing will miss this completely.

What this feels like: your hair starts thinning in your 20s or 30s, even though nothing else feels wrong. You might be told you have “genetic male pattern baldness” or “genetic female pattern baldness,” which is true, but nobody explains that your genetics loaded the gun. This variant is the reason why. If you carry the sensitive variant, DHT blockers tend to work better for you than they do for people with less sensitive receptors.

The SRD5A2 gene encodes an enzyme called 5-alpha reductase type 2, which sits inside your hair follicle cells and converts testosterone into DHT. DHT is the potent form of the hormone that drives hair follicle miniaturization. If the AR gene is the lock, SRD5A2 is the key-maker. Your SRD5A2 status determines how much DHT your follicles are actually producing.

The V89L variant (rs523349), carried by roughly 30-40% of people, affects the enzyme’s efficiency. Some variants increase DHT production; others slightly decrease it. The key point is that two people with identical testosterone levels can have completely different DHT production in their hair follicles depending on their SRD5A2 variant. Your bloodwork will show normal testosterone. It won’t show hair follicle DHT, which is what actually matters.

What this feels like: if you carry a variant that increases DHT production, your hair loss tends to progress faster and earlier than you’d expect from your overall hormone levels. You might be responding well to a DHT-blocking medication while a friend on the same dose sees no benefit. This is often a SRD5A2 difference. If you carry a variant that slightly decreases DHT production, you might respond better to other interventions that don’t directly target DHT.

Estrogen is a protective hormone for hair. It lengthens the growth phase of the hair cycle and delays the transition to the shedding phase. The ESR1 gene encodes the estrogen receptor, the protein that allows your hair follicles to respond to circulating estrogen. If your estrogen receptor is insensitive, your follicles can’t benefit from estrogen protection, even if your estrogen levels are normal.

The PvuII and XbaI variants in ESR1, carried by roughly 40% of people, reduce estrogen receptor sensitivity. This means your hair follicles simply don’t respond as well to estrogen, so you lose the protective effect that estrogen normally provides. This is especially relevant for women experiencing hair loss during perimenopause or menopause, when estrogen drops. But it also affects men, because they produce estrogen too, and they benefit from its hair-protective effects.

What this feels like: if you have this variant, hormonal changes hit your hair harder than they hit other people’s. A pregnancy might cause dramatic shedding postpartum because your follicles can’t leverage the rapid hormone shifts the way other follicles can. Menopause triggers accelerated hair loss. Even fluctuations in your menstrual cycle might be noticeable in your hair. Standard hormone testing shows that your estrogen level is normal, but your follicles are locked out of receiving estrogen’s protective signal.

MTHFR is an enzyme that plays a central role in the methylation cycle, which is the cellular process that regenerates tissue and maintains DNA stability. Hair follicles are among the fastest-dividing cells in your body. They require a massive amount of methylation capacity to rebuild themselves during the growth phase. If your MTHFR enzyme is inefficient, your hair follicles literally run out of the raw materials needed to regenerate.

The C677T variant, carried by roughly 40% of people of European ancestry, reduces MTHFR enzyme efficiency by 30-40%. This means your cells are slower at processing B vitamins into the methylated forms that drive the regeneration process. Your bloodwork might show normal folate and B12 levels, but your follicle cells are functionally depleted. They have the raw ingredients but not the enzyme capacity to use them.

What this feels like: diffuse hair thinning across the entire scalp, rather than patterned hair loss. You might notice that your hair is weaker, breaks more easily, and grows more slowly. Biotin and B-complex vitamins might help a little, but never as much as you’d expect. This is because you need the methylated forms of these vitamins, not the standard forms. Standard bloodwork shows B12 and folate are fine. Your cells are still starving for them.

The VDR gene encodes the vitamin D receptor, a protein that allows your cells to respond to vitamin D. In your hair follicles, vitamin D is essential for transitioning follicles from dormancy into the active growth phase. Without proper VDR function, your follicles get stuck. They don’t activate properly, so new hair doesn’t grow to replace what’s being shed. You’re losing hair but not replacing it at the normal rate.

The BsmI and FokI variants, carried by roughly 30-50% of people, reduce VDR function. This means your hair follicles don’t respond optimally to vitamin D, even if your vitamin D blood level is technically normal. Your follicles are waiting for a vitamin D signal that never comes strongly enough to trigger regrowth. This is why some people with “normal” vitamin D levels still see dramatic improvements in hair growth when they supplement with higher doses of vitamin D.

What this feels like: your hair loss might be accompanied by other signs of vitamin D receptor insensitivity, like slow wound healing, difficulty building muscle, or even seasonal mood changes. Your vitamin D blood level comes back at 35 ng/mL (technically normal), but your hair keeps falling out. Increasing your vitamin D intake doesn’t help until you push your level significantly higher than standard recommendations. This variant explains why.

The HFE gene regulates iron absorption. Iron is essential for hair follicle health, but too much iron is toxic to hair follicles (and every other cell). HFE maintains the balance. If your HFE gene has a variant, your iron metabolism might be off. Either you’re absorbing too much iron, which accelerates hair loss through oxidative stress, or your iron handling is inefficient, leading to deficiency. Both extremes damage hair.

The C282Y and H63D variants, carried by roughly 5-10% of people of European ancestry, affect iron absorption. Many people with these variants never develop clinical hemochromatosis (iron overload disease), but they do have altered iron metabolism that affects hair. Your iron bloodwork might look normal, but your cells might be sitting in a state of chronic oxidative stress from excess iron, or struggling to access the iron they need.

What this feels like: if you have an HFE variant that increases iron absorption, your hair might be thinning despite supplementing with iron. You might feel fatigued, achy, or notice joint pain alongside hair loss. If your variant affects iron utilization, you might have persistently low-normal iron levels that your doctor says are fine, but your follicles are struggling. Standard iron supplementation doesn’t help because the problem isn’t iron quantity, it’s your ability to use it.