Your Testosterone Is Normal. Your Genes May Not Be.

The androgen receptor is the lock on your muscle cells. Testosterone is the key. When testosterone binds to this receptor, it tells your muscles to grow, tells your bone to strengthen, and tells your brain to feel motivated. Without this receptor, testosterone can’t signal anything, no matter how high your levels are.

AR gene variants are determined by something called a CAG repeat length. The longer the repeat, the less sensitive your androgen receptor is to testosterone. Think of it as a lock that’s increasingly hard to turn. A man with a long CAG repeat can have high testosterone levels but still experience weak muscle gain, low motivation, and fatigue because his cells simply aren’t responding to the signal.

If you carry a longer CAG repeat, you’ve been trying to build muscle and boost energy while fighting a cellular communication problem. Your muscles aren’t ignoring the testosterone signal because they don’t want to respond. They’re not responding because the receptor is working at reduced efficiency. This is why generic testosterone replacement often fails for men with AR variants. You need either higher doses, a different form of hormone, or interventions that bypass the receptor problem entirely.

SHBG is the delivery van for testosterone in your blood. Testosterone doesn’t float around freely. It binds to SHBG, which carries it through the bloodstream. Only the unbound testosterone can actually enter muscle cells and trigger growth. The SHBG gene determines how much SHBG your body produces. Carry certain variants and your SHBG levels run high, which means more testosterone gets bound and unavailable.

Roughly 30 to 40 percent of men carry SHBG variants that increase SHBG production. When SHBG runs high, you can have normal total testosterone but critically low free testosterone, leaving almost nothing available for your muscles to use. It’s like having money in a locked safe instead of in your pocket. Technically you’re wealthy, but functionally you’re broke.

You’ve experienced this. You train hard, eat enough protein, sleep well, and still can’t build muscle. Your total testosterone might even be above average. But your free testosterone is in the basement because SHBG is binding it all. This explains why some men feel dramatically better simply lowering their SHBG through targeted interventions, even without changing their total testosterone at all.

Aromatase is the enzyme that converts testosterone into estrogen. Your body needs this conversion to happen, but it needs to happen at the right rate. Too much conversion and you lose testosterone while estrogen climbs. Too little and you lose the benefits of estrogen (bone density, cardiovascular health). The CYP19A1 gene determines how active your aromatase is. Variants in this gene shift the conversion rate up or down.

Carriers of certain CYP19A1 variants run high aromatase activity. This means they convert testosterone to estrogen at an accelerated rate, leaving less testosterone available for muscle growth and leaving them with an estrogen-dominant state that tanks energy, libido, and motivation. This is common enough that roughly 20 to 30 percent of men experience meaningful aromatase dysregulation as a contributor to low testosterone symptoms.

If you’ve noticed that adding testosterone replacement made you feel worse, not better, high aromatase may be the reason. You gained testosterone but immediately lost most of it to estrogen conversion. This is why some men need aromatase inhibition or dietary changes that reduce aromatase activity. Without understanding your CYP19A1 status, you could take testosterone for years and never benefit.

COMT clears dopamine from your brain. Dopamine drives motivation, sexual desire, and the drive to move your body and build muscle. When COMT is fast, dopamine clears quickly and you feel less motivated. When COMT is slow, dopamine accumulates and you feel more driven. The COMT Val158Met variant determines your dopamine clearance rate. Roughly 25 percent of people in European ancestry populations carry the slow-clearance variant homozygously.

Slow COMT carriers accumulate dopamine, which typically feels good: more motivation, better focus, stronger sexual desire. Fast COMT carriers clear dopamine quickly, leaving them chronically undermotivated, less interested in sex, and struggling to find the drive to train hard and consistently. A man with fast COMT can have perfectly normal testosterone and still experience crushing motivational flatness because dopamine, not testosterone, drives the will to act.

This explains why two men can have identical testosterone levels but experience completely different energy and motivation. One has fast COMT and feels like he’s pushing through mud just to get to the gym. The other has slow COMT and finds himself naturally drawn to hard training. Genetics isn’t destiny, but it does shape the motivational landscape you’re working with.

Vitamin D doesn’t do anything unless it binds to the VDR, the vitamin D receptor. This receptor sits on muscle cells, bone cells, immune cells, and testosterone-producing cells. When vitamin D binds to VDR, it activates testosterone synthesis in the testes and also increases how responsive muscle cells are to testosterone’s signal. The VDR gene determines how efficiently this binding occurs. Common variants in VDR can reduce receptor sensitivity, meaning you need higher vitamin D levels to achieve the same biological effect.

Men with VDR variants often find that their testosterone levels improve when they get vitamin D into an optimal range, but they need higher absolute levels than men with wild-type VDR. A VDR variant carrier might need 60 to 80 ng/mL of vitamin D to feel the full testosterone-boosting effect, while someone with normal VDR sees benefits at 40 to 50 ng/mL. This is why generic vitamin D supplementation sometimes fails. You’re taking the standard dose and hitting the standard target, but your genetics require more.

Beyond vitamin D status, VDR also modulates how muscle cells respond to testosterone signaling. A VDR variant means you’re starting from a disadvantage on two fronts: lower testosterone production and lower muscle responsiveness to the testosterone you do produce. This is a compounding problem that bloodwork won’t reveal.

MTHFR controls methylation, a chemical process that happens millions of times per second in your cells. One critical methylation product is BH4, a cofactor required to make nitric oxide. Nitric oxide is the gas that relaxes blood vessel walls, allowing blood to flow freely. Your muscles need blood flow to grow. Your penis needs blood flow to function. Your brain needs blood flow to think clearly. The MTHFR C677T variant, carried by roughly 40 percent of people in European ancestry populations, reduces MTHFR enzyme activity by 40 to 70 percent.

When MTHFR is impaired, BH4 production drops, nitric oxide synthesis suffers, and blood vessel function declines. This means less blood reaches your muscles during training, slower nutrient delivery, slower waste removal, and slower recovery. You can train hard and eat right, but if your blood vessel function is compromised, muscle growth hits a ceiling. Additionally, impaired methylation means impaired thyroid function, impaired hormone metabolism, and impaired ability to process the B vitamins that support testosterone production.

Many men with MTHFR variants think their problem is hormonal. The real problem is vascular. Their muscles aren’t getting enough blood during training. Their recovery is compromised. Their hormone metabolism is sluggish. Generic testosterone therapy bypasses the root cause, which is why results plateau. Fixing the methylation pathway often unlocks the testosterone response.