Detailed definition
Transdermal drug absorption requires the molecule to traverse the stratum corneum (the keratinized outer skin layer) before entering the dermal capillaries. Steroid hormones like estradiol have favorable physical chemistry for transdermal delivery (lipophilic, low molecular weight), enabling the routine clinical use of patches, gels, and sprays. Once steady-state delivery is established, plasma estradiol levels remain relatively constant over the dosing interval (a 7-day patch produces roughly stable levels for the duration). Application factors that influence absorption include skin temperature (higher temperature increases absorption — a hot bath or fever can transiently raise levels from a gel), hydration, application site (abdomen and buttock are standard for patches; arm and thigh for gels), shaving and broken skin, and concurrent use of other topicals. Patch transfer to others (children, partners) is uncommon with proper application but possible.
Why it matters in menopause
Practical absorption considerations for HRT users: rotate patch sites to reduce skin irritation, allow gel to dry before clothing or skin contact, avoid applying to broken skin, and recognize that very hot environments (sauna, hot tub) can transiently raise levels.
Related terms
Sources
External references: Wikipedia.