Your HRT Isn't Working the Way It Should. Your Genes May Be Why.

Your estrogen receptors are the locks on your cells that estrogen unlocks. ESR1 controls how many of these locks you have and how easily estrogen can fit into them. This is the foundational step in whether estrogen therapy will work at all. Some women have receptors that are very responsive; others have fewer receptors or receptors that don’t bind estrogen as tightly. If your ESR1 variant creates lower receptor sensitivity, you’re essentially trying to flood the system with a hormone that your cells are not primed to receive.

The PvuII and XbaI variants in ESR1 are carried by roughly 40% of women in European ancestry populations. These variants reduce estrogen receptor sensitivity, meaning your cells respond less dramatically to the same dose of hormone therapy. This is not about the amount of estrogen in your blood; it’s about whether your cells care when estrogen shows up.

You might notice that you need higher doses than other women to feel the same relief. Or you might feel almost nothing from a standard dose, even though your blood levels look normal. Hot flashes may persist despite adequate serum estrogen. Mood may not lift the way it should. Vaginal dryness might not improve. Brain fog remains. When ESR1 sensitivity is low, your body is essentially asking for more hormone, or for a different strategy altogether.

Aromatase is the enzyme that sits at the crossroads of male and female hormones. It converts testosterone into estrogen in your ovaries, fat tissue, and other organs. This conversion is critical: if you don’t have enough aromatase activity, you can’t convert the testosterone your body makes into the estrogen you need, even if you’re taking HRT. If you have too much aromatase activity, estrogen can build up too high. Either way, your hormone balance is off.

CYP19A1 variants are common in the population and affect aromatase enzyme activity across a range. Depending on your variant, you may be over-converting or under-converting testosterone to estrogen, leaving you with either too little estrogen or too much relative to testosterone. A standard HRT dose doesn’t account for this difference. If you’re an under-converter, adding more exogenous estrogen helps, but only if your system can keep up. If you’re an over-converter, the same dose might push estrogen levels dangerously high.

You might experience wildly fluctuating symptoms even on a stable HRT dose. Hot flashes might swing between intense and absent. Mood might be unpredictably unstable. Bleeding patterns (if you still have a uterus) might be erratic. Your estrogen levels on blood work might not match how you feel because the enzyme conversion is either running too fast or too slow, creating imbalance regardless of what you’ve taken.

COMT is a cleanup enzyme. It clears estrogen, testosterone, dopamine, and norepinephrine from your system. If COMT is fast, hormones and stress chemicals get cleared quickly, which is usually good for stress resilience but can mean hormones don’t stick around long enough to do their job. If COMT is slow, hormones linger in your system, which sounds good until they linger too long and create side effects. COMT speed is determined by a single amino acid switch at position 158: valine (fast) or methionine (slow).

Roughly 25% of people of European ancestry are homozygous for the slow COMT variant (Met/Met). Slow COMT means estrogen, testosterone, and dopamine stay in your bloodstream longer, and you’re more sensitive to their effects. For HRT, this can be a double-edged sword. On one hand, you might need a lower dose because the hormone lingers and has more time to work. On the other hand, you’re more prone to side effects: breast tenderness, mood swings, anxiety, insomnia, or migraine.

If you have slow COMT, you might feel great on a dose that makes other women feel nothing. Or you might feel anxious, wired, or have mood swings even on a low dose because the hormones are accumulating faster than your system can tolerate. You might also be sensitive to caffeine or other stimulants because your COMT struggles to clear dopamine and norepinephrine as well. Your HRT symptoms might improve on one dose, then worsen as the hormone accumulates over weeks.

MTHFR is not about folate itself; it’s about methylation,the chemical process your cells use to make, regulate, and clear hormones. Your body clears estrogen through methylation in the liver and kidneys. If your MTHFR is slow, methylation backs up, and estrogen clears slower than it should. Slow MTHFR also impairs your ability to synthesize the molecules that help regulate estrogen receptor sensitivity and hormone metabolism. This creates a cascade problem: hormones linger, receptor sensitivity gets dysregulated, and you’re stuck in a cycle where standard HRT doses don’t work.

The C677T variant in MTHFR is carried by roughly 40% of people in European ancestry populations. This variant reduces enzyme efficiency by 40 to 70 percent, meaning your cells can’t methylate estrogen and other hormones effectively, causing them to accumulate and create side effects. This is different from not having enough folate in your diet; this is a structural problem with how your enzyme works.

You might notice that you feel worse on HRT even though your blood levels are normal, or that you develop new symptoms like migraines, mood swings, or breast tenderness after starting. You might have had good responses to supplements or medications in the past, but HRT feels different, like your body can’t process it. Fatigue might worsen because methylation is energy-intensive and your system is already strained. You might feel cognitively foggy because impaired methylation affects neurotransmitter metabolism too.

Vitamin D is not just a vitamin; it’s a hormone regulator. Your vitamin D receptors sit on cells throughout your body and control how sensitively your cells respond to estrogen, testosterone, and other hormones. If your VDR is inefficient, you don’t absorb and use vitamin D well, even if your blood levels look adequate. This means your cells lose some of their ability to calibrate their hormone sensitivity. Low VDR function also affects immune tolerance, which matters because excess estrogen from HRT can trigger autoimmune flares in susceptible women.

VDR variants like FokI are common in the population; roughly 50% of people carry at least one variant allele. VDR variants reduce your cells’ ability to activate and use vitamin D, meaning even adequate serum levels fail to support proper hormone receptor function and immune regulation. This is why some women have low vitamin D on blood work, and others have normal levels but still experience symptoms that suggest deficiency.

You might find that you can’t tolerate higher estrogen doses because your immune system becomes hypersensitive. You might develop new joint pain, inflammation, or autoimmune-like symptoms after starting HRT. Your bones might not respond to HRT the way they should because vitamin D is essential for estrogen’s bone-protective effects. You might have low vitamin D despite supplementing because your VDR variant limits how much you can absorb and activate it.

SHBG is the carrier protein in your bloodstream that binds to estrogen and testosterone. When a hormone is bound to SHBG, it’s not available to do anything; it’s just being transported. Only the free, unbound hormone can enter your cells and activate receptors. If your SHBG is high, most of your hormone is bound and stuck, leaving very little circulating free. If your SHBG is low, more hormone is free, but you also have less carrier capacity, which can lead to rapid clearance or metabolism. SHBG levels are partly genetic and partly influenced by insulin, liver health, and thyroid function.

The rs6259 and rs1799941 variants in SHBG are carried by roughly 30 to 40% of people, and they influence how much SHBG your liver produces. Genetic variants that increase SHBG mean more of your HRT hormone gets bound up and unavailable, even though your serum hormone levels look normal. This is the opposite of bioavailable hormone. Your blood work might show adequate estrogen, but functionally, your cells are starving for it.

You might need higher HRT doses because so much of the hormone is bound to SHBG that almost none reaches your cells. You might feel like your symptoms improved briefly after starting, then plateaued, because initial free hormone was higher but as steady state was reached, binding took over. You might have poor energy, low libido, or persistent hot flashes despite what looks like adequate serum levels. Your mood might not improve because the free hormone available to cross the blood-brain barrier is too low.