Oral Estradiol Equivalent to 0.037 mg/day Transdermal Patch
A 0.037 mg/day (37 mcg/day) transdermal estradiol patch is approximately equivalent to 0.75-1 mg of oral micronized estradiol daily.
Dose Conversion Rationale
The established conversion between transdermal and oral estradiol demonstrates that 100 mcg/day transdermal estradiol is approximately equivalent to 2 mg oral micronized estradiol daily 1. Using this conversion:
- 37 mcg/day transdermal ÷ 100 mcg/day = 0.37
- 0.37 × 2 mg oral = 0.74 mg oral estradiol
Therefore, the equivalent oral dose would be approximately 0.75-1 mg daily of oral micronized estradiol 1.
Critical Prescribing Considerations
Pharmacokinetic Differences
Oral estradiol undergoes extensive first-pass hepatic metabolism, converting largely to estrone, whereas transdermal delivery maintains more physiologic estradiol-to-estrone ratios 2, 3. This means:
- Oral doses of 1 mg/day result in estrone concentrations that markedly exceed premenopausal reference ranges 3
- Transdermal estradiol has a neutral effect on venous thromboembolism risk (OR 0.9), whereas oral estradiol significantly increases VTE risk (OR 4.2) 1
- Blood pressure and metabolic profiles are more favorable with transdermal versus oral estradiol 1
Mandatory Endometrial Protection
Women with an intact uterus must receive progestin supplementation when taking either transdermal or oral estradiol to prevent endometrial hyperplasia and cancer 1, 4. The recommended regimen is:
- Oral micronized progesterone 200 mg daily for 12-14 days every 28 days (sequential regimen) 1, 4
- Alternative: 10 mg medroxyprogesterone acetate for 12-14 days monthly 4
- Failure to add progestin is a critical prescribing error that can lead to endometrial cancer 4
Common Pitfalls to Avoid
Do Not Assume Direct Equivalence
The conversion is approximate and subject to significant interindividual variation 5. One study found that 24.84% of women using the highest licensed transdermal dose still had subtherapeutic estradiol levels (<200 pmol/L) 5. This variation is even greater with oral administration due to variable first-pass metabolism 3.
Consider Cardiovascular Risk Profile
If switching from transdermal to oral estradiol, counsel patients about the increased cardiovascular and thrombotic risks associated with oral administration 1. The transdermal route is strongly preferred for patients with:
- History of or risk factors for venous thromboembolism 1
- Cardiovascular disease risk factors 1
- Hypertension or metabolic concerns 1
Avoid Overdosing with Oral Estradiol
Conventional oral doses of 1-2 mg/day result in urinary estrone excretion at values 5-10 times the upper limit of premenopausal reference ranges 3. Starting with 0.75-1 mg oral estradiol for a 37 mcg/day patch equivalent is appropriate, but some evidence suggests even lower doses (0.25 mg/day) may be prudent to avoid excessive estrone exposure 3.
Titration Based on Clinical Response
Approximately 77% of patients require no adjustment from their initial dose, but dose customization is key 6, 5. Monitor symptom control and consider measuring serum estradiol levels if symptoms persist or if there are concerns about absorption 5.