What is an aromatase inhibitor?
An aromatase inhibitor is a medication that blocks the enzyme aromatase, preventing the conversion of testosterone to estrogen in the body.
Aromatase inhibitors (AIs) are a class of medications that block the enzyme aromatase, which is responsible for converting testosterone into estrogen. Common aromatase inhibitors used in testosterone replacement therapy include anastrozole, letrozole, and exemestane. When exogenous testosterone is administered during TRT, some converts to estrogen through aromatization, particularly in adipose tissue. This can lead to elevated estrogen levels, causing side effects such as gynecomastia (breast tissue enlargement), water retention, mood changes, and reduced sexual function.
By inhibiting the aromatase enzyme, these medications reduce estrogen conversion and help maintain a favorable testosterone-to-estrogen ratio. This approach allows patients to achieve therapeutic testosterone levels while minimizing estrogen-related side effects. Aromatase inhibitors are typically used selectively during TRT rather than routinely with every patient, as some degree of estrogen is necessary for bone health, cardiovascular function, and sexual performance. The decision to use an AI, and which one, depends on individual aromatization rates, baseline estrogen levels, and clinical response to therapy.
Providers offering testosterone replacement therapy often assess estrogen levels through bloodwork to determine whether an aromatase inhibitor is appropriate for a given patient, since overuse can suppress estrogen to levels that create their own complications.