Pharmacokinetics of estrogen

The main types of pharmacologic therapy used to treat the hormone deficiency of menopause are parenteral and oral administrations of estrogen. Parenteral administration results in predictable absorption without major intermediary metabolism, whereas all oral estrogens are subject to intestinal metabolism before entering the systemic circulation. Various options are available. Injectable estrogens, because of rapid absorption and metabolism, are impractical for long-term replacement therapy. The primary drawback to subcutaneous estradiol pellets is the surgical procedure required for their insertion and removal. Vaginal epithelium is an effective pathway for absorption of estrogen given by solution, tablet, or cream, although only the latter is currently approved for clinical use; relatively constant serum levels of estradiol can be obtained with vaginal rings. Transdermal patches provide controlled hormone levels for up to 3.5 days. Although oral conjugated equine estrogens have been used extensively, they introduce types of estrogen, such as equilin, that are not naturally found in humans and that can produce a pronounced hepatic response. Micronized estradiol, oral estrone, and other estrogens have been used as alternatives to equine estrogens. Clinicians should understand the pharmacokinetics of the various options for replacement therapy and select a course of treatment that is safe, effective, and convenient for the patient.