Estrogens act through two nuclear receptor subtypes: estrogen receptor alpha and estrogen receptor beta. Both are ligand-activated transcription factors that, upon estrogen binding, move into the nucleus and regulate gene expression. The two subtypes differ in their tissue distribution, and this difference is the pharmacological foundation for selective estrogen receptor modulators.

Estrogen receptor alpha predominates in the uterus, breast, liver, bone, and the hypothalamic-pituitary axis. It mediates the negative feedback of estradiol on gonadotropin secretion. Estrogen receptor beta is expressed preferentially in the ovary, colon, lung, and central nervous system, and its activation tends to oppose the proliferative effects driven by estrogen receptor alpha in tissues where both receptors are present.

Estrogen enters cells and binds the estrogen receptor in the cytoplasm or nucleus. The receptor-ligand complex dimerizes, binds to specific deoxyribonucleic acid sequences in the promoter regions of target genes, and activates or represses transcription. This genomic pathway takes hours to produce effects because it requires new messenger ribonucleic acid synthesis and protein production. Rapid membrane-associated estrogen effects (within minutes) also exist but are less relevant to Step 1 pharmacology.

Natural progesterone binds the progesterone receptor selectively with minimal activity at the androgen, glucocorticoid, or mineralocorticoid receptors. Synthetic progestins were designed to improve oral bioavailability but retained structural features that produce off-target receptor binding. This cross-reactivity explains the major adverse effect differences between progestin generations: progestins derived from testosterone (such as levonorgestrel) bind the androgen receptor and can worsen lipid profiles and cause acne. Progestins derived from spironolactone (such as drospirenone) block both the androgen receptor and the mineralocorticoid receptor, producing anti-androgenic and potassium-retaining effects respectively.

Estradiol is the bioidentical form of the primary ovarian estrogen. When taken orally, it undergoes extensive first-pass metabolism in the intestinal mucosa and liver, where it is converted to the weaker estrone and then to inactive sulfate conjugates. The portal circulation delivers high estrogen concentrations directly to the liver, stimulating hepatic synthesis of sex hormone-binding globulin, coagulation factors, angiotensinogen, and inflammatory proteins — effects that increase venous thromboembolism risk.

Transdermal estradiol (patches, gels, sprays) bypasses this first-pass hepatic exposure. Estradiol absorbed through the skin enters the systemic circulation directly, reaching the liver at normal physiological concentrations rather than as a concentrated portal bolus. The result is minimal stimulation of hepatic coagulation and inflammatory proteins. Observational data consistently show that transdermal estradiol does not increase venous thromboembolism risk even at higher doses, making it the preferred route in women with risk factors for clotting. The rule is straightforward: oral estrogen increases venous thromboembolism risk; transdermal estradiol does not.

Ethinyl estradiol is a synthetic estrogen designed for oral use. A chemical modification at the C-17 position blocks the hepatic enzyme that rapidly degrades natural estradiol, giving ethinyl estradiol a much higher oral bioavailability than estradiol. Despite this improved stability, ethinyl estradiol is still a potent hepatic stimulant: even at the low doses used in modern combined oral contraceptives (20 to 35 micrograms per day), it produces substantially greater hepatic estrogenic stimulation than any transdermal preparation. This hepatic potency is the reason combined oral contraceptive use increases venous thromboembolism risk approximately three to four times above baseline.

Conjugated equine estrogens (Premarin) are a mixture of water-soluble estrogen sulfates extracted from pregnant mares' urine. The mixture includes several estrogens not naturally present in humans, including equilin and equilenin, which have longer biological activity than estradiol. Conjugated equine estrogens were the estrogen used in the Women's Health Initiative trial — they are not interchangeable with transdermal estradiol in interpreting trial risk data.

Estetrol is a native fetal estrogen now available in a combined oral contraceptive paired with drospirenone. It has lower hepatic estrogenic stimulation than ethinyl estradiol, with early data suggesting a more favorable venous thromboembolism and metabolic profile, though long-term comparative outcome data are still accumulating.

The most pharmacologically important kinetic distinction for estrogens is the presence or absence of first-pass hepatic exposure. Oral estradiol and ethinyl estradiol both expose the liver to high concentrations via the portal circulation, stimulating hepatic production of sex hormone-binding globulin, coagulation factors, C-reactive protein, and angiotensinogen. Transdermal estradiol bypasses this exposure and does not produce these hepatic effects at therapeutic doses. This single difference determines the venous thromboembolism risk difference between oral and transdermal estrogen formulations.

Vaginal estrogen preparations (creams, suppositories, low-dose rings) are used for genitourinary syndrome of menopause and act primarily locally with minimal systemic absorption at standard doses, making them generally suitable even for women with contraindications to systemic estrogen.

Ethinyl estradiol in combined oral contraceptives potently induces hepatic sex hormone-binding globulin production, raising circulating levels two to four times above baseline. Because testosterone is transported bound to sex hormone-binding globulin, this induction reduces the free (biologically active) testosterone fraction. This pharmacodynamic effect is the basis for prescribing ethinyl estradiol-containing combined oral contraceptives in women with androgen-excess conditions such as polycystic ovary syndrome, acne, and hirsutism.

Androgenic progestins such as levonorgestrel compete with testosterone for sex hormone-binding globulin binding, partially countering this effect and increasing free testosterone. Anti-androgenic progestins such as drospirenone do not, making drospirenone-containing pills preferable in androgen-excess conditions.

Depot medroxyprogesterone acetate (150 mg intramuscular every 12 weeks) achieves contraceptive serum concentrations within 24 hours of injection and maintains them for 3 months. A high-yield counseling point: the average time to return of fertility after discontinuing depot medroxyprogesterone acetate is 9 to 10 months, substantially longer than for any other reversible contraceptive method. Patients planning pregnancy in the near term should be counseled about this delay before starting the injection.

The etonogestrel subdermal implant (Nexplanon) provides contraceptive concentrations for 3 years from a single insertion and is the most effective reversible contraceptive available. Fertility returns within 3 to 4 weeks of implant removal.