HRT Estrogen Route Conversion

How this works

Estrogen formulations are rarely studied head-to-head, so dose equivalence is always approximate. This calculator anchors its conversions to the best available consensus: the CMS Systemic MHT Equivalency Table (January 2025), cross-referenced against concordant guidelines from the British Menopause Society, ACOG, and the Australasian Menopause Society. Where those sources agree, the conversion factors are as reliable as pharmacokinetic data allows.

BiEst cream conversions use the Sood et al. 2013 pharmacokinetic study, which randomized 40 women to compounded BiEst 80:20 cream versus Vivelle-Dot patch and measured steady-state serum estradiol. BiEst 80:20 at 3.0 mg total (containing 0.6 mg estradiol and 2.4 mg estriol) produced serum estradiol levels that were not statistically different from a 0.05 mg/day patch. Lower BiEst doses (2.0 mg and 2.5 mg) produced lower estradiol levels than the patch, confirming that topical cream has lower and more variable bioavailability than patch delivery.

Transdermal estradiol (patches, gels, creams) bypasses first-pass liver metabolism. This matters clinically: in the Vinogradova 2019 BMJ analysis of over 80,000 cases, transdermal HRT showed no statistically significant increase in venous thromboembolism risk, while oral estrogen carried a 40 to 73% increased risk depending on formulation. Transdermal delivery also avoids the hepatic increase in sex hormone-binding globulin, thyroxine-binding globulin, triglycerides, and C-reactive protein that oral estrogen produces. For women with elevated BMI, migraine with aura, personal or family VTE history, or gallbladder disease, transdermal is the preferred route.

Estriol, the weaker estrogen in BiEst formulations, has roughly 10% of estradiol's binding affinity at ER-alpha receptors and a shorter receptor residence time. It preferentially activates ER-beta receptors, which are concentrated in urogenital and brain tissue. Some evidence suggests partial agonist/antagonist activity in breast tissue, though this remains an area of active research. When switching from BiEst to a non-BiEst formulation, the estriol component is lost; standard serum estradiol labs do not reflect estriol's separate receptor activity.

Progesterone matching is required for anyone with a uterus taking systemic estrogen. Unopposed estrogen increases endometrial cancer risk 10- to 30-fold after five years. Micronized progesterone (Prometrium) is preferred over synthetic progestins based on current society guidelines and evidence of a more favorable breast and cardiovascular safety profile. The dose scales with the estrogen dose: higher systemic estradiol exposure requires higher progesterone for reliable endometrial protection.

Individual variation is real. Skin thickness, body composition, ambient temperature, cream base formulation, application site, and even time since showering all affect transdermal absorption. This is why the calculator shows ranges rather than single numbers, and why serum estradiol should be checked 6 to 8 weeks after any formulation switch.

References

Canadian Menopause Society. Systemic Menopause Hormone Therapy (MHT) Equivalency Table. CMS; 2025.

Sood R, Warndahl RA, Schroeder DR, et al. Bioidentical compounded hormones: a pharmacokinetic evaluation in a randomized clinical trial. Maturitas. 2013;74(4):375-382.

Vinogradova Y, Coupland C, Hippisley-Cox J. Use of hormone replacement therapy and risk of venous thromboembolism: nested case-control studies using the QResearch and CPRD databases. BMJ. 2019;364:k4810.

National Academies of Sciences, Engineering, and Medicine. The Clinical Utility of Compounded Bioidentical Hormone Therapy: A Review of Safety, Effectiveness, and Use. Washington, DC: National Academies Press; 2020.

The Menopause Society (NAMS). The 2022 hormone therapy position statement of The North American Menopause Society. Menopause. 2022;29(7):767-794.