Is an elevated estradiol level in a man receiving testosterone therapy concerning, given that he previously used anastrozole (Arimidex) only on injection days, and how should the aromatase inhibitor regimen be adjusted?

An estradiol level of 87 pg/mL in a man on testosterone therapy is elevated and warrants intervention, though the intermittent anastrozole dosing regimen (only on injection days) is suboptimal and likely contributing to inadequate estradiol control.

Understanding the Clinical Context

The established guidelines for testosterone therapy monitoring do not specifically address estradiol management, as the Endocrine Society guidelines 1 focus primarily on monitoring testosterone levels, hematocrit, PSA, and prostate health. However, the AUA guideline 2 recommends measuring serum estradiol in testosterone-deficient patients who present with breast symptoms or gynecomastia prior to commencing therapy, acknowledging that estradiol monitoring has clinical relevance.

Is This Estradiol Level Concerning?

Yes, an estradiol of 87 pg/mL is elevated and clinically significant. While there is no universally agreed-upon upper limit for estradiol in men on testosterone therapy, research demonstrates that:

• Men with elevated estradiol levels on testosterone therapy may develop symptomatic gynecomastia, breast tenderness, and other estrogen-related side effects 2
• The typical target range for estradiol in men is generally considered to be 20-40 pg/mL, though this varies by laboratory and clinical context
• Recent data shows that men with estradiol >60 pg/mL or 40-60 pg/mL with symptoms are commonly treated with aromatase inhibitors 3

The Problem with Intermittent Anastrozole Dosing

The patient's previous regimen of taking anastrozole only on testosterone injection days is pharmacologically inappropriate. Here's why:

• Anastrozole has a half-life of approximately 50 hours 4, meaning it takes about 7 days to reach steady-state plasma concentrations
• Plasma concentrations approach steady-state levels at about 7 days of once-daily dosing 4
• Intermittent dosing prevents achievement of stable aromatase inhibition, resulting in fluctuating estradiol levels that mirror the testosterone peaks and troughs from injections
• The FDA-approved dosing for anastrozole is 1 mg daily (for breast cancer in postmenopausal women), and off-label use in men typically employs consistent dosing schedules 4

Recommended Management Strategy

Implement a consistent anastrozole dosing schedule rather than intermittent use:

1. Start with anastrozole 0.5 mg three times weekly (e.g., Monday/Wednesday/Friday) 3

   • This is the most commonly studied regimen in men with elevated estradiol on testosterone therapy
   • In a study of 44 men with elevated estradiol on testosterone therapy, this regimen reduced median estradiol from 65 pg/mL to 22 pg/mL without affecting testosterone levels 3

2. Recheck hormone levels in 4-6 weeks:

   • Measure both estradiol AND total testosterone
   • Critical pitfall: Excessive aromatase inhibition can paradoxically worsen symptoms and harm bone health 5, 6
   • Target estradiol range: 20-40 pg/mL (though some sources suggest up to 60 pg/mL may be acceptable if asymptomatic)

3. Adjust anastrozole dose based on response:

   • If estradiol remains >60 pg/mL: increase to 1 mg three times weekly
   • If estradiol drops <20 pg/mL: reduce frequency to twice weekly or 0.25 mg three times weekly
   • If estradiol 20-40 pg/mL and asymptomatic: continue current dose

Important Caveats and Monitoring

Monitor for adverse effects of aromatase inhibition:

• Bone health is a major concern: Aromatase inhibition in older men decreases bone mineral density 5

  • In a randomized trial, anastrozole decreased spine BMD from 1.121 to 1.102 g/cm² over one year 5
  • Consider baseline DEXA scan if planning long-term anastrozole use, particularly in men >50 years

• The goal is estradiol optimization, not suppression: Estradiol is essential for bone health, sexual function, and cardiovascular health in men 7

• Testosterone levels must remain stable: Ensure testosterone stays in the mid-normal range (400-700 ng/dL) 1

Factors Predicting Response to Anastrozole

Men most likely to respond favorably to anastrozole therapy 8:

• Baseline testosterone-to-LH ratio ≥100
• Non-azoospermic at baseline
• Higher baseline testosterone levels
• Lower baseline LH and FSH levels

Notably, baseline estradiol level and estradiol-to-testosterone ratio were NOT predictive of treatment response 3, 8, suggesting that the absolute estradiol value of 87 pg/mL alone doesn't determine whether anastrozole will be effective—but it does indicate treatment is warranted.

Alternative Considerations

If the patient is using intramuscular testosterone injections:

• Consider switching to more frequent injections (e.g., twice weekly instead of weekly) to reduce peak-to-trough fluctuations
• Transdermal formulations may result in lower estradiol levels compared to injectable testosterone 3
• In the study by Punjani et al., 38.6% of men requiring anastrozole were on intramuscular testosterone versus only 34.1% on topical formulations 3

Body composition matters:

• Obesity increases aromatase activity in adipose tissue
• Weight loss may reduce estradiol levels without medication 9
• Consider addressing underlying metabolic factors

Bottom Line Algorithm

For this patient with estradiol 87 pg/mL on testosterone therapy:

1. Discontinue the "injection day only" anastrozole regimen
2. Start anastrozole 0.5 mg three times weekly on a fixed schedule
3. Recheck estradiol and testosterone in 4-6 weeks
4. Titrate anastrozole dose to achieve estradiol 20-40 pg/mL while maintaining testosterone 400-700 ng/dL
5. Monitor for symptoms of excessive estrogen suppression (joint pain, decreased libido, erectile dysfunction)
6. Consider DEXA scan if planning >1 year of anastrozole therapy