Your Testosterone Is Dropping, But Your Genes May Be Why.

The AR gene codes for the androgen receptor, the protein that sits on your cells and listens for testosterone. When testosterone binds to this receptor, it triggers the entire cascade of male sexual and metabolic traits: muscle growth, libido, mood stability, bone density, and metabolic rate. Without a functioning androgen receptor, testosterone is just noise.

The AR gene has a variable section called a CAG repeat. The number of repeats varies between roughly 8 and 35 in the human population. This repeat length is common across different ancestry groups, but the frequency of longer repeats varies. The problem is that longer CAG repeats mean a less sensitive androgen receptor. Your cells literally hear testosterone signals more quietly. A man with 20 CAG repeats might feel fantastic on moderate testosterone levels. A man with 28 repeats, with the exact same testosterone level, might feel depleted.

You notice this as a mismatch between your testosterone number and how you feel. Your energy is lower than it should be for your testosterone level. Your muscle builds slowly despite heavy training. Your libido doesn’t match the testosterone reading. Your mood is flatter than you’d expect. None of this is in your head. Your receptor is just less sensitive, and it needs higher testosterone levels to achieve the same effect.

SHBG is a transport protein. It binds testosterone and estrogen in the bloodstream and carries them around. This sounds helpful, but SHBG is also a trap. When SHBG binds your testosterone, that testosterone can’t enter your cells. It’s locked away. Only the unbound fraction, called free testosterone, can actually do anything. You can have a total testosterone level that looks perfectly healthy on paper and have almost none of it available to your cells.

The SHBG gene has several common variants. Roughly 30 to 40 percent of the population carries the variants that increase SHBG production. These are often inherited from your parents and influenced by lifestyle factors like liver health and body composition. The variants that raise SHBG do exactly what the name suggests: they increase SHBG production, which binds up more of your testosterone and estrogen, making both less bioavailable. You’re left with normal total testosterone but low free testosterone, and your cells never get the signal.

This variant has a distinctive symptom pattern. Your energy is dragging. Your libido is low or gone. You might have signs of relative estrogen dominance, like water retention or soft tissue accumulation around the midsection, even though your total testosterone is in range. You feel depleted despite normal blood work. That’s SHBG working against you.

Aromatase is the enzyme that converts testosterone into estrogen. This is supposed to happen. Men need some estrogen for bone health, cardiovascular function, and mood. But some men’s genes make aromatase hyperactive. They produce too much of the enzyme, and too much testosterone gets shunted into estrogen. This is particularly common in men with obesity or with certain genetic variants in the CYP19A1 gene itself.

The CYP19A1 gene has several common variants that are found across populations. These variants affect how much aromatase your cells produce. When you carry the variant associated with higher aromatase activity, your cells are biased toward converting testosterone to estrogen rather than preserving testosterone. You might have perfectly normal testosterone production, but your body is converting it away faster than it should be, leaving you with low free testosterone and relatively high estrogen. This is especially true if you also carry the variants that increase SHBG or reduce androgen receptor sensitivity.

The symptom pattern here is distinctive: low energy and libido paired with signs of estrogen dominance like breast tissue growth, water retention, mood instability, and sometimes erectile dysfunction. Your testosterone might be normal, even high, but your estrogen-to-testosterone ratio is inverted. You feel like you’re running on estrogen, because you are.

COMT is an enzyme that breaks down dopamine, norepinephrine, and epinephrine. This is a necessary process. You don’t want these neurotransmitters building up forever. But the speed at which COMT works varies tremendously between people, and much of that variation is genetic. Some people are fast metabolizers. Their COMT works like a cleanup crew on overtime. Other people are slow metabolizers. Their COMT works like a sleepy security guard.

The COMT gene has a common variant called Val158Met. Roughly 25 percent of people with European ancestry are homozygous for the slow version. The slow version means your body clears dopamine more slowly, so dopamine levels stay elevated longer. This might sound like a gift. It isn’t. Elevated dopamine without proper clearance actually reduces dopamine receptor sensitivity, which means your cells hear the dopamine signal less clearly, not more. You feel less motivated, less driven, less interested in reward-seeking behaviors like sex. Your libido is mysteriously low despite normal testosterone. Your motivation is flat.

This is made worse by the fact that dopamine is central to erectile function and sexual arousal. When your dopamine signaling is suppressed, everything downstream of reward and arousal gets suppressed with it. You might not have low testosterone at all. You might have a COMT variant that’s dampening your dopamine sensitivity, which is dampening your sex drive, which is making you feel like you have low testosterone.

Vitamin D isn’t really a vitamin. It’s a hormone precursor. Your skin makes it from sunlight. Your liver and kidneys activate it. And then it binds to the vitamin D receptor, the VDR protein, which sits on your cells and controls the expression of hundreds of genes, including genes involved in testosterone synthesis, androgen receptor expression, and immune regulation. Without a functioning VDR, vitamin D signals are lost in translation.

The VDR gene has several common variants, including the Fok1, Bsm1, Apa1, and Taq1 polymorphisms. These are found across all populations. The variants that code for a shorter Fok1 VDR protein are associated with greater vitamin D sensitivity, while the longer versions are associated with reduced sensitivity. Roughly 40 to 50 percent of the population carries variants associated with lower VDR sensitivity. This means your cells don’t respond as efficiently to vitamin D signaling, even if your vitamin D levels are normal. You’re vitamin D resistant at the cellular level, not the blood level.

Because vitamin D is involved in testosterone synthesis and androgen signaling, VDR variants that reduce vitamin D sensitivity indirectly reduce your body’s ability to make and respond to testosterone. Your vitamin D blood test might look fine. Your testosterone might test normal. But your cells aren’t getting the signal. You feel depleted, your mood is lower, your immune system is under-responsive, and your testosterone response is muted despite adequate serum levels.

MTHFR is an enzyme that converts folate into its active form, methylfolate. Methylfolate is the currency of the methylation cycle, the biochemical pathway that controls everything from gene expression to neurotransmitter synthesis to hormone metabolism. If your MTHFR is working poorly, your methylation cycle is sluggish, and testosterone metabolism gets stuck. You can’t efficiently convert testosterone into other metabolic forms. You can’t efficiently produce the cofactors needed for androgen receptor expression. You’re essentially running on biochemical fumes.

The MTHFR C677T variant is carried by roughly 40 percent of people with European ancestry. The T variant codes for an enzyme that works at only 40 to 70 percent of normal efficiency. This is a huge difference at the cellular level. Your body can’t generate enough methyl groups to properly metabolize testosterone, support androgen receptor expression, or maintain the nitric oxide production needed for erectile function. You’re not short on testosterone. You’re short on the biochemistry to use it.

The symptom pattern here is subtle but pervasive. Your energy is low and doesn’t improve with rest. Your libido is flat and unresponsive to lifestyle changes. Your mood is unstable or depressed. Your blood work looks mostly normal, but you feel chronically depleted. You might also have signs of elevated homocysteine, poor methylation, or difficulty tolerating B vitamins. That’s your MTHFR struggling to keep up with your metabolic demands.