Estrogen and Progestin Pharmacology

Gonadal Pharmacology · Module 1 of 5

Estrogens act through two nuclear receptor subtypes — ERα (uterus, breast, bone, liver) and ERβ (ovary, CNS, colon) — whose tissue distribution explains the tissue-selective pharmacology of SERMs.
Ovarian estrogen synthesis requires two cooperating cell types: theca cells generate androgens under LH drive, and granulosa cells convert them to estrogens via aromatase (CYP19A1) under FSH control — neither cell type can produce estradiol alone.
Oral estrogens undergo portal first-pass metabolism that amplifies hepatic coagulation factor production and raises VTE risk; transdermal estradiol bypasses the portal circulation and does not increase VTE risk — a clinically decisive route difference.
Progestin generation determines receptor cross-reactivity: testosterone-derived progestins (norethindrone, levonorgestrel) are androgenic; spironolactone-derived drospirenone is anti-androgenic and anti-mineralocorticoid; natural micronized progesterone is progesterone-receptor selective.
CYP3A4-inducing drugs — especially rifampin and enzyme-inducing antiepileptic drugs — reduce hormonal contraceptive efficacy; the lamotrigine–ethinyl estradiol interaction is bidirectional and can precipitate either seizures or toxicity depending on which drug is started or stopped.

Estrogen Receptors

Estrogen Receptor Alpha

Dominant in Reproductive Tissues & Liver

Uterus, breast, liver, bone
Hypothalamic-pituitary axis (negative feedback)
Target for tamoxifen and raloxifene (SERMs)
Mediates proliferative and feminizing effects

Estrogen Receptor Beta

Modulates Estrogen Receptor Alpha Activity

Ovary (granulosa cells), colon, lung, central nervous system
Generally opposes proliferative effects of estrogen receptor alpha
Less targeted by available drugs
Basis for tissue-selective SERM pharmacology

Estrogen Biosynthesis — Two-Cell, Two-Gonadotropin Model

Theca Cell

LH → androgens (androstenedione, testosterone) from cholesterol

Granulosa Cell

FSH → aromatase (CYP19A1) → converts androgens to estrogens

Products

Androstenedione → estrone | Testosterone → estradiol (most potent)

Postmenopause

Ovary stops; estrone from adipose aromatization of adrenal androstenedione becomes dominant → aromatase inhibitors work here

Estrogen Route — VTE Risk Comparison

Property	Oral Estradiol	Ethinyl Estradiol	Transdermal Estradiol
First-pass hepatic	Yes — high portal concentrations	Yes — even higher hepatic stimulation	No — bypasses portal circulation
Coagulation factors	Increased	Substantially increased	Minimal increase
SHBG induction	Moderate increase	2–4× increase	Minimal
VTE risk	Increased	~3–4× baseline	Not increased
Preferred in VTE risk	No	No	Yes

Progestin Generations — Receptor Profiles

Natural

Micronized Progesterone

Selective for progesterone receptor
Micronization improves oral bioavailability
Sedation via GABA-A metabolites
Vaginal use: luteal phase support

1st / 2nd Gen 19-Nor

Norethindrone & Levonorgestrel

Testosterone-derived → androgenic activity
Levonorgestrel: highest androgenic index
Reduces HDL; acne risk
Depot MPA: 150 mg q12 wks; fertility delay 9–10 mo

3rd Gen 19-Nor

Desogestrel / Etonogestrel

Reduced androgenic activity vs. levonorgestrel
Etonogestrel: implant (3 yr), vaginal ring
Better lipid profile
Slightly higher VTE risk than levonorgestrel

4th Gen / Spiro-Derived

Drospirenone

Anti-androgenic (androgen receptor blocker)
Anti-mineralocorticoid → K+ retention
Monitor K+ with ACE-I, ARBs, K+-sparing diuretics
Use: polycystic ovary syndrome, PMDD

Key Drug Interactions

CYP3A4 Inducers → Contraceptive Failure

Rifampin & Enzyme-Inducing Antiepileptic Drugs

Rifampin: most potent inducer; renders all hormonal OCs unreliable
Induction persists 4 weeks after last rifampin dose
Enzyme-inducing AEDs: carbamazepine, phenytoin, phenobarbital, oxcarbazepine
Safe AEDs: valproate, levetiracetam, gabapentin, pregabalin
Backup: copper intrauterine device (non-hormonal)

Bidirectional Interaction

Ethinyl Estradiol + Lamotrigine

Ethinyl estradiol induces lamotrigine glucuronidation
Start combined OC → lamotrigine levels fall 40–65% → seizure risk
Stop combined OC → lamotrigine levels rebound → toxicity (dizziness, diplopia)
Progestin-only methods: no interaction (no ethinyl estradiol)
Preferred: progestin-only or non-hormonal method in lamotrigine-treated epilepsy

Core Rule — Oral vs. Transdermal Estrogen

Oral estrogen (ethinyl estradiol, oral estradiol, conjugated equine estrogens) → portal first-pass → hepatic coagulation factors up → VTE risk up. Transdermal estradiol bypasses the portal circulation → no VTE risk increase. In patients with VTE risk factors, the transdermal route is always preferred.

Suggested References

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th ed. — Chapter 40: Estrogens, Progestins, and the Female Reproductive Tract	McGraw-Hill; 2021
Brunton L, Knollmann B, Hilal-Dandan R, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 44: Estrogens and Progestins	McGraw-Hill; 2023
Heldring N, Pike A, Andersson S, et al.	Estrogen receptors: how do they signal and what are their targets	Physiol Rev. 2007;87(3):905–931
Nilsson S, Makela S, Treuter E, et al.	Mechanisms of estrogen action	Physiol Rev. 2001;81(4):1535–1565
Levin ER.	Membrane oestrogen receptor alpha signalling to cell functions	J Physiol. 2009;587(21):5019–5023
Schindler AE, Campagnoli C, Druckmann R, et al.	Classification and pharmacology of progestins	Maturitas. 2008;61(1–2):171–180
Drummond AE, Fuller PJ.	The importance of ERbeta signalling in the ovary	J Endocrinol. 2010;205(1):15–23
Simpson ER, Davis SR.	Minireview: aromatase and the regulation of estrogen biosynthesis	Endocrinology. 2001;142(11):4589–4594
Kuhnz W, Blode H, Zimmermann H.	Pharmacokinetics of exogenous natural and synthetic estrogens and antiestrogens	In: Estrogens and Antiestrogens II. Springer; 1999:261–322
Canonico M, Oger E, Plu-Bureau G, et al; ESTHER Study Group.	Hormone therapy and venous thromboembolism among postmenopausal women: impact of the route of estrogen administration and progestogens	Circulation. 2007;115(7):840–845
Stanczyk FZ, Hapgood JP, Winer S, Mishell DR Jr.	Progestogens used in postmenopausal hormone therapy: differences in their pharmacological properties, intracellular actions, and clinical effects	Endocr Rev. 2013;34(2):171–208
Tavaniotou A, Smitz J, Bourgain C, Devroey P.	Comparison between different routes of progesterone administration as luteal phase and early pregnancy support	Hum Reprod Update. 2000;6(2):139–148
van den Heuvel MW, van Bragt AJ, Alnabawy AK, Kaptein MC.	Comparison of ethinylestradiol pharmacokinetics in three hormonal contraceptive formulations: the vaginal ring, the transdermal patch and an oral contraceptive	Contraception. 2005;72(3):168–174
Simmons KB, Haddad LB, Nanda K, Curtis KM.	Drug interactions between non-rifamycin antibiotics and hormonal contraception: a systematic review	Am J Obstet Gynecol. 2018;218(1):88–97