Oral and Transdermal Estrogen Dose Equivalents

Prescribing estrogen replacement therapy (ERT) can be confusing. When ERT first came to market in 1942 it was a simple choice, as there was only one FDA-approved medication, Premarin. As the name indicates, it is derived from Pregnant Mares Urine, a mixture of conjugated horse estrogens. Today, plant-based and “bioidentical estrogens,” which have the same chemical and molecular structure as human estrogen, are also available.

      Even with the availability of newer forms of ERT, Premarin remains the most prescribed form of ERT worldwide. Practitioners are most comfortable writing for Premarin, a drug they are very familiar with, as it has been available for more than 70 years. Additionally, information on how to convert oral Premarin into other oral or transdermal therapies has not been readily available.

      The following chart compares oral and transdermal estrogens and summarizes the names, formulations, dosages, and routes of administration and most importantly, dose equivalents, for available oral and transdermal ERT options.

• Plasma estrogen peaks & troughs

• First pass through GI tract & liver (requires higher dose)

• Increased hepatic enzymes, inflammatory markers

• Increased triglycerides

• Increased blood pressure

• Reduced IGF-1

• Decreased LDL cholesterol and increased HDL

• Serum E2 levels relatively constant

• Does not pass through liver – lower doses required

• No change in inflammatory markers

• No change or decrease in triglycerides

• Decrease in blood pressure

• No effect on GH/IGF-1

• Decreases LDL but no change in HDL