What aromatase is
Aromatase (gene name CYP19A1) is a cytochrome P450 enzyme that catalyzes the conversion of androgens (testosterone, androstenedione) to estrogens (estradiol, estrone). Specifically, it aromatizes the A-ring of the steroid molecule, adding an aromatic chemical structure that converts androgen to estrogen.
Where it's expressed
- Adipose tissue (the major peripheral site)
- Brain (hypothalamus, hippocampus, others)
- Bone
- Gonads (testes, ovaries)
- Placenta (during pregnancy)
- Skin
- Some other tissues
Different tissues use locally produced estradiol for different purposes, brain estradiol affects cognition; bone estradiol protects density; etc.
What regulates it
- Adipose mass, more fat = more aromatase capacity
- Insulin, modulates expression
- Cortisol, can upregulate aromatase
- Inflammatory cytokines, upregulate adipose aromatase
- Age, peripheral aromatase activity may increase with age
- Substrate availability, more T → more conversion
Implications for men
In men:
- Most circulating estradiol comes from peripheral aromatization, not testicular production
- Obesity drives proportionally higher E2/T ratio
- High E2 from excess aromatization feeds back to suppress LH and lower T further (the hypogonadism-obesity cycle)
- Reducing body fat reduces aromatase activity, lowers E2, raises T
- This is the mechanism by which weight loss raises testosterone in obese men
Implications for women
In women:
- Ovarian aromatase produces estradiol from theca cell androgens (the major source premenopausally)
- Postmenopausally, peripheral aromatase becomes the dominant E2 source
- Adipose aromatase produces relatively more estrone than estradiol
- This is part of why postmenopausal obesity is associated with breast and endometrial cancer risk
Aromatase inhibitors
Pharmacological AIs:
- Anastrozole (Arimidex), non-steroidal, reversible
- Exemestane (Aromasin), steroidal, irreversible
- Letrozole (Femara), non-steroidal, more potent
Uses:
- Postmenopausal hormone-receptor-positive breast cancer (primary indication)
- TRT-related estrogen excess in men (selective use; often overprescribed)
- PCOS (occasional)
- Idiopathic short stature in boys (off-label)
Aromatase deficiency
Congenital aromatase deficiency (rare) shows what happens without estrogen production:
- Severe osteoporosis despite normal T
- Failure to fuse epiphyses (continued linear growth)
- Insulin resistance, obesity
- Lipid abnormalities
- Reproductive abnormalities
This pattern reveals what aromatase normally provides, and what aggressive AI use can compromise.
The clinical pearl: Aromatase isn't a "problem" to be eliminated. It's an essential enzyme producing essential estradiol. Pharmacological inhibition is appropriate in specific contexts (breast cancer, severe E2 excess on TRT) but problematic when overused.
Bottom line
Aromatase converts androgens to estrogens. The enzyme is essential for bone, brain, vascular, and other functions. Higher body fat increases aromatization. Pharmacological inhibitors have specific uses but are often overprescribed. Understanding the enzyme explains many hormone interaction patterns.