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Guide · Gene explainer

Does genetics affect how much testosterone converts to oestrogen?

Aromatase is the enzyme that turns testosterone into oestradiol. Its gene, CYP19A1, carries the strongest genetic influences on men’s oestradiol found so far. Here is what large studies show, and why it matters less than you might expect.

Quick answer

Yes, modestly. Variants in the aromatase gene CYP19A1 are the strongest genetic influences on oestradiol in men, but each shifts oestradiol by around 5 to 10%. Body fat, age and the amount of testosterone present change oestradiol far more, and no gene test predicts gynaecomastia.

  • rs2470152 was the top oestradiol variant among 604 tested in sex-hormone genes, replicated in three cohorts.
  • A study of 11,097 men found three independent CYP19A1 signals for oestradiol.
  • UK guidance expects oestradiol above 150 pmol/L in people using steroids, whatever their genes.
Testosterone is converted to oestradiol by aromatase (CYP19A1) and to DHT by 5-alpha reductase (SRD5A2)TestosteroneOestradiolDHTAromatasegene: CYP19A15-alpha reductasegene: SRD5A2Acts through the androgen receptor (gene: AR)
Gene
CYP19A1, on chromosome 15. Makes aromatase.
What aromatase does
Converts testosterone to oestradiol, and androstenedione to oestrone, in fat, muscle, brain, bone and the testes.
Strongest variant in men
rs2470152 (intron 1), for oestradiol; replicated.
Size of effect
Around 5 to 10% difference in oestradiol per variant or haplotype.
Can't predict
Gynaecomastia, water retention, or what any substance will do.

What large studies found

In 2009, Swedish researchers tested 604 variants across 50 sex-hormone genes in 1,041 young men. The variant most strongly linked to oestradiol was rs2470152, in the first intron of CYP19A1, and the link was confirmed in two further cohorts of older men, 5,531 men in all.[1] A 2018 genome-wide study of 11,097 men found that variants in or near CYP19A1 had the strongest effect on oestradiol of anywhere in the genome, resolving to three independent signals.[2]

The effects are modest. In the Framingham Heart Study, each copy of one CYP19A1 haplotype that includes rs700518 went with about 5% higher oestradiol, 17% lower testosterone and a 24% higher oestradiol-to-testosterone ratio in men.[3] Across a large prostate cancer consortium, several CYP19A1 variants were linked to 5 to 10% differences in oestradiol.[4]

Why it matters less on steroids

Aromatase converts whatever testosterone is there. Taking testosterone raises oestradiol well above a few per cent: UK primary-care guidance expects oestradiol above 150 pmol/L (about 41 pg/mL) in people using anabolic steroids, notes it may cause breast tissue growth and changes in libido, and advises against aromatase inhibitors.[5] A 5% genetic difference is small next to that.

What it can’t tell you

Your CYP19A1 genotype can’t tell you whether you will develop gynaecomastia, how you will respond to any substance, or whether steroids are safe for you.

The other enzyme: 5-alpha reductase

Testosterone is also converted to DHT by 5-alpha reductase, made by SRD5A2. A common variant, V89L, reduces the enzyme’s activity in laboratory studies, but whether it measurably changes DHT in people is unresolved.[6] See the overview of hormone genes.

Questions

Does genetics affect how much testosterone converts to oestrogen?

Yes, a little. Variants in and near CYP19A1, the aromatase gene, are the strongest genetic influences on oestradiol in men found so far, but the differences are around 5 to 10% per variant. Body fat, age and how much testosterone is present matter far more.

Which CYP19A1 variants affect oestradiol?

In men, rs2470152 was the variant most strongly linked to oestradiol in a screen of 604 sex-hormone gene variants, replicated in two further cohorts. A study of 11,097 men later found three independent signals in the gene. A haplotype that includes rs700518 was linked to about 5% higher oestradiol per copy in the Framingham study.

Can a gene test predict gyno on steroids?

No. Taking testosterone raises oestradiol far more than any aromatase variant does: UK guidance expects oestradiol above 150 pmol/L in people using anabolic steroids. No genetic test has been shown to predict who develops gynaecomastia.

Should I take an aromatase inhibitor because of my genes?

No. UK primary-care guidance advises against aromatase inhibitors for people using anabolic steroids, and no genetic result changes that. Talk to a GP about any breast tissue changes.

Is CYP19A1 in my 23andMe data?

Several studied CYP19A1 variants usually are. Open your file in our raw DNA checker to see your genotypes, with what each was studied for.

Sources

  1. Eriksson AL et al. Genetic variations in sex steroid-related genes as predictors of serum estrogen levels in men. J Clin Endocrinol Metab 2009;94:1033–41.
  2. Eriksson AL et al. Genetic determinants of circulating estrogen levels and evidence of a causal effect of estradiol on bone density in men. J Clin Endocrinol Metab 2018;103:991–1004.
  3. Peter I et al. Variation in estrogen-related genes associated with cardiovascular phenotypes and circulating estradiol, testosterone, and DHEA sulfate levels. J Clin Endocrinol Metab 2008;93:2779–85.
  4. Travis RC et al. CYP19A1 genetic variation in relation to prostate cancer risk and circulating sex hormone concentrations in men. Cancer Epidemiol Biomarkers Prev 2009;18:2734–44.
  5. Gibbons SM et al. Essential blood testing in the patient using androgenic anabolic steroids. Br J Gen Pract 2024;74:187–90.
  6. Makridakis N et al. A prevalent missense substitution that modulates activity of prostatic steroid 5alpha-reductase. Cancer Res 1997;57:1020–22.

For information only, not medical advice. GeneticEdge does not recommend or endorse anabolic steroid use, and no genetic test can tell you whether steroids are safe for you. Talk to a GP or pharmacist about your health. Last reviewed 30 October 2026.