Anastrozole (Arimidex) for Men on Testosterone Therapy

Most men think of estrogen as exclusively a female hormone, but men also produce it. In fact, testosterone is the precursor of estradiol, an estrogen, and the primary female sex hormone. Testosterone is converted into estradiol via the aromatase enzyme.

Because testosterone can be converted into estradiol, taking testosterone via testosterone replacement therapy (TRT) can potentially elevate estrogen levels. Fortunately, most men on standard TRT dosages do not experience high estradiol levels. Nevertheless, genetically predisposed men may experience symptoms associated with high levels, which may lead to certain unwanted side effects.

What is the Role of Estradiol in Men?

Many men assume that estradiol is simply “bad”. They think that because estradiol is one of the primary female hormones and because high levels may cause gynecomastia and/or edema, estradiol should be reduced as much as possible. However, this is not true.

While high levels cause certain side effects in genetically susceptible, estradiol plays many important roles in men. For example, it plays an important role in determining bone mineral density in men. Low levels may well be the major cause of bone loss in elderly men.

Low estradiol levels are associated with increased fat and decreased sexual desire and erectile function compared to higher levels.

What are High Estradiol Levels in Men?

The definition of high estradiol levels in men varies by laboratory and type of analytical method. Unfortunately, there is also no consensus on what type of test (regular or ultrasensitive) is best.
Some labs use an electrochemiluminescence immunoassay methodology and define high estradiol as equal to or above 42.6 pg/mL.

Side Effects of High Estradiol Levels in Men

As mentioned, testosterone replacement therapy dosages that do not raise testosterone above normal generally do not lead to high estradiol levels. Consequently, the feminizing effects associated with high levels are generally not seen with TRT dosages. Unfortunately, genetically predisposed men on TRT may experience symptoms associated with high estrogen levels.
Symptoms of high estradiol levels in men include two of the most common side effects of testosterone therapy. These include 1) gynecomastia (male breast development and enlargement) and 2) edema (fluid retention in the extremities).

Gynecomastia is breast enlargement in men due to benign (non-cancerous) breast tissue growth. Breast tissue contains receptors for estrogen. As such, high levels of estrogens promote breast tissue growth and lead to gynecomastia. Unfortunately, medical treatment of gynecomastia that has persisted beyond a year is often ineffective. After one year, most cases must be treated via surgical excision.

Edema is soft tissue swelling caused by fluid retention. Fluid retention may occur in the arms and legs at the beginning of testosterone therapy. It generally resolves after the first few months of treatment, if treatment dosages are reduced, or if treatment is stopped. Nonetheless, edema may persist and may cause pain.

Co-administration of Anastrozole (Arimidex)

The Endocrine Society’s Clinical Practice Guidelines for Testosterone Therapy in Adult Men with low testosterone does not address the potential issue of high estradiol levels during TRT. As such, The Endocrine Society does not recommend for or against any treatments that lower estradiol levels. Also, there is currently a lack of guidelines as to the critical level whereby intervention is needed to treat high levels.

Notwithstanding the foregoing, some physicians do treat men on TRT with high estradiol levels by co-administering anastrozole (brand name Arimidex) with TRT at 0.05 – 1.0 mg every 1 – 3 days. Anastrozole is an aromatase-inhibiting drug that comes in the form of a pill and is taken orally. It inhibits the ability of the aromatase enzyme to convert testosterone to estrogen. Thus, it lowers the level of circulating estradiol quite effectively.

Anastrozole is actually FDA-approved for the treatment of breast cancer in women after surgery. As such, its use in men on testosterone replacement therapy is considered to be off-labeled.