Estradiol pharmacology

Reference synthesis · inline C1–C5 confidence tags · cross‑linked to evidence and reference.

1Molecule and metabolism

17β‑estradiol (C₁₈H₂₄O₂, MW 272.4) is a four‑ring steroid (A, B, C, D) with two metabolically reactive hydroxyls. The C3 hydroxyl is a phenol on the aromatic A ring (pK_a ~10.7, so essentially fully protonated and uncharged at physiologic pH; log P ~4.0, high passive permeability).C1 The C17 hydroxyl is a 17β‑configured aliphatic secondary alcohol on the saturated D ring — chemistry that admits oxidation to the C17 ketone (E1) by removing the carbinol hydrogen.

Enzymes that act on E2

Directional pumps

The HSD17B1/HSD17B2 pair and the SULT1E1/STS pair each form a tissue‑directional pump. Liver and gut express the “off” arms (HSD17B2 with NAD⁺; SULT1E1) and inactivate E2 on absorption. Ovary, breast, brain, endometrium, and bone express the “on” arms (HSD17B1 with NADPH; STS) and reactivate E1S and E1 to E2. The cytoplasmic cofactor pools enforce this directionality — fed‑state hepatocyte [NAD⁺]/[NADH] ~700 (Williamson 1967) drives oxidation; cytoplasmic [NADPH]/[NADP⁺] ~100 in target tissues drives reduction.C1 Plasma E1/E2 ~5 at oral steady state is the empirical readout of this hepatocyte equilibrium (the thermodynamic calculation predicting an intracellular ratio of ~11 has correct arithmetic but fragile cofactor inputs, so the observed plasma ratio is the load‑bearing anchor).C3

Mass balance for oral 1 mg E2

Of an oral 1 mg dose, ~98% is absorbed across the enterocyte (high passive permeability). Gut‑wall conjugation by UGT1A10 removes an inferred ~30–50% of absorbed E2 before portal entry.C3 Of the residual that reaches the liver, ~90% is extracted on first pass (HSD17B2 oxidation to E1, SULT1E1 sulfation to E1S, UGT glucuronidation with biliary export via MRP2). Only ~40 µg (~4% of dose) exits the liver as free E2 — the canonical “F_oral ≈ 5%” figure.C1 The v4 PK model partitions the remaining ~96% into ~30% E1, ~32% E1S, ~29% glucuronides, and ~5% catechols/E3/methylated metabolites; these fractions are model calibration outputs, not measured human radiolabel data (no portal‑vein cannulation study has been done on E2 the way Back & Rogers 1982 did for EE).C3 Kuhl 2005's textbook fractions (~15% E1, ~25% E1S, ~50% glucuronides) are looser anchors of the same shape.C2

The qualitative load‑bearing point: only a few percent of an oral E2 dose becomes free plasma E2; most becomes circulating E1 and E1S that target tissues reactivate over the following 12–24 h. The “5% bioavailability” figure understates total estrogenic activity over time, and overstates the dose actually presented to peripheral receptors.

2Pharmacokinetics by route

Route determines two distinct quantities: how much parent drug reaches systemic circulation (F), and how much the hepatocyte sees on first pass (the portal‑vein concentration during absorption). These can diverge dramatically. Oral and sublingual deliver the dose through portal vein; transdermal, IM ester depot, and SC depot bypass it.

The E1:E2 ratio is a route signature. Premenopausal cycling produces ~1:1 (the ovary makes both, peripheral conversion adds a little). Transdermal and injectable routes give ~1:1 because they bypass the portal vein. Oral routes give 5:1 or higher because hepatic HSD17B2 runs the NAD⁺‑driven equilibrium during first‑pass.C1

Oral E2

Free‑E2 plasma bioavailability is ~5% for the parent drug; the apparent plasma half‑life is 13–20 h, much longer than the free‑E2 IV half‑life of 1–2 h.C1 The discrepancy is the E1S reservoir slowly back‑feeding the free pool via STS reactivation (§4). Steady‑state plasma E1S on 1 mg/d is 2,000–4,000 pg/mL (~50–100× the E2 level).C1 E2‑valerate is hydrolyzed to E2 + valeric acid by gut‑wall and plasma esterases within ~30 min, then behaves identically to oral E2 (MW correction: E2V is ~76% E2 by mass).

Sublingual E2

Some fraction of a sublingual dose absorbs across the buccal/sublingual mucosa directly to systemic circulation as free E2, bypassing portal first‑pass; the rest is swallowed and follows the oral route. C_max is ~3–4× oral at the same doseC1 and the curve is distinctly biphasic: a sharp ~1.7 h early decline reflecting free‑E2's true IV‑like clearance, then a slower late tail driven by the smaller E1S reservoir from the swallowed fraction.C2 The absolute direct‑sublingual bioavailability has never been cleanly measured in humans. The v4 model uses ~5% (calibrated against Doll 2022's AUC ratio of 1.8× oral); older textbooks cite 20–25%; the only primary anchor for absolute F is the Kuhnz 1993 marmoset study at ~10%.C2

Sublingual is not reliably liver‑sparing. Cirrincione 2021 (LC‑MS/MS) finds sublingual produces plasma E1 levels closer to oral than to transdermal — meaning the swallowed fraction and the slowly redistributing direct fraction both hit hepatocytes.C1 Bar 2024 (ECE conference abstract, EP592) reports sublingual E2 produces meaningful free protein S depression, similar in magnitude to oral.C4

Aggregated mean curves from Burnier 1981, Casper & Yen 1981, Fiet 1982, Kuhnz 1993, Price 1997, Wiegratz 2001, Wren 2003, Pickar 2015 — single‑dose 0.25–2 mg micronized estradiol. Sublingual gives a sharp 1–2 h peak followed by fast biphasic decay; oral gives a broad ~8 h peak with smoother decline. Inter‑study C_max spread for 1 mg SL is ~100–400+ pg/mL — a factor of 4. From transfemscience.org sublingual review.

Transdermal

Transdermal patch and gel deliver E2 directly to systemic circulation through skin, bypassing both gut and liver first‑pass. A 50 µg/d patch produces ~50–80 pg/mL steady state with E1:E2 ~1:1 and minimal SHBG induction. Hepatic exposure equals systemic exposure, so the receptor sees free E2 at sub‑K_d levels at typical doses — well below the SHBG EC50.C3

Injectable (IM, SC)

IM ester depots (E2‑valerate t½ ~2 d release, E2‑cypionate t½ ~6.5 d, E2‑undecylate t½ ~22 d) hydrolyze in the depot and slowly release free E2 into systemic circulation. Single‑dose IM E2V 5 mg peaks at ~667 pg/mL around day 2; IM EC 5 mg at ~338 pg/mL around day 4 (peak values commonly understated as ~200/~100 pg/mL in earlier drafts of this synthesis — those are C_avg, not C_peak). SC E2 aqueous and depot routes are increasingly used in trans HRT for self‑injection convenience; PK matches the IM equivalent ester.

Oral esters (E2V, EC, EUn)

Oral E2‑valerate is essentially oral E2 from minute 30 onward; oral EC and EUn are less commonly used orally (the longer aliphatic chains slow ester hydrolysis but most clinical use is parenteral depot). The European HRT and COC market (Climen, Qlaira) uses E2V as the oral E2 source by convention; the US market uses unesterified E2.

3Hepatic ER‑α and SHBG

Hepatic ER‑α (ESR1) is the molecular mediator of route‑dependent estrogen effect on the liver. When activated, it transcriptionally upregulates coagulation factors II, VII, VIII, IX, X, XI, fibrinogen, SHBG, CBG, TBG, angiotensinogen, IGFBPs, HDL, and triglycerides; and downregulates protein S, antithrombin, and IGF‑1. The net hemostatic shift is procoagulant, and it tracks the concentrations the hepatocyte sees — not systemic plasma E2.

The two curves

The single most important framing correction from round‑2 fact‑checking: receptor occupancy and SHBG induction are governed by two different dose‑response curves at very different concentrations. Earlier drafts conflated them under one Hill curve at K_d = 0.25 nM (~68 pg/mL free), concluding “routes below ~68 pg/mL avoid hepatic effects.” This is too crisp.

• ER‑α occupancy tracks free hepatocyte E2 with K_d ~0.25 nM (Vickers et al. 1989, rat hepatocyte).C1 Recombinant human ER‑α sits at K_d 0.05–0.5 nM, so the cross‑species applicability is defensible.
• SHBG mRNA induction in HepG2 cells responds to E2 in the 0.5–2.5 µM range (Kalme 1999), three to four orders of magnitude above the receptor K_d.C1 The mechanism: transcriptional activation requires sustained occupancy and downstream ESR1‑cofactor recruitment, not just transient binding.
• The v4 PK model uses ~1500 pg/mL (~5.5 nM) free hepatic E2 as the SHBG EC50 — a calibration target chosen so the model's predicted oral‑E2 dose‑response (1 mg → +60%, 2 mg → +120%, 4 mg → ~+180% SHBG) lands inside the measured +67–171% band Ropponen et al. 2005 reported for oral E2 in humans (the 1 mg point is the model's, not the study's — that trial used oral E2 valerate 2–4 mg/d and published no per‑dose breakdown).C1 This is not a measured EC50, and it should NOT be cited to Selva & Hammond 2009 — that paper is on HNF‑4α/thyroid‑hormone regulation of SHBG, not E2 dose‑response. Earlier drafts contained this misattribution.C1

The two timescales

The “two curves” above separate occupancy from induction along the concentration axis. They are also separated along the time axis, and the time gap is what makes the protein readout sluggish. A single oral dose floods the hepatocyte for only minutes during first‑pass, but the protein it ultimately changes — SHBG, clotting factors — turns over across days. The signal therefore passes through a ladder of increasing time constants:

• Ligand–receptor binding is diffusion‑limited and reaches its binding equilibrium in well under a second; with a sub‑nanomolar K_d (~0.25 nM, Vickers 1989) the on/off cycling is fast relative to everything downstream.C3
• Receptor occupancy — the fraction of ER‑α bound at any instant — tracks free hepatocyte E2 essentially in real time (seconds–minutes), so it rises and falls with the first‑pass concentration spike.C3
• Transcriptional output (ESR1‑cofactor recruitment → RNA Pol II → mRNA accumulation) builds over hours; it integrates the occupancy signal rather than mirroring its instantaneous value.C3
• Protein output is the slowest step. Secreted SHBG accumulates in plasma against a multi‑day clearance (terminal t½ ~4 days in primate tracer studies, ~7 days commonly cited clinically), so plasma SHBG reaches a new steady state only after about two weeks (3–4 × the ~4‑day half‑life) — well after the dose that drove it has cleared.C2

The hepatic exposure rule

Plasma protein levels track the concentrations the hepatocyte sees during absorption, not the systemic E2 level. Oral E2 at 1 mg/d produces ~50 pg/mL plasma E2 but ~5000 pg/mL hepatic free E2 during first‑pass — comparable hepatic exposure to pregnancy (~300 pg/mL sustained free hepatic) despite 1/500 the systemic level. The pregnancy comparison is the cleanest natural experiment for the “route, not dose” claim: pregnancy plasma E2 is ~250× cycling baseline, yet pregnancy VTE risk is only ~3–10× baseline — the attenuation comes from how the dose reaches the liver (placenta‑to‑systemic, not portal‑vein bolus).C1

Plasma protein binding and the free fraction

SHBG enters the story twice. The previous subsection covered SHBG as a hepatic transcriptional output (the protein the liver makes more of under oral‑style first‑pass exposure). It is also the dominant carrier for E2 in plasma, and the two roles couple: raising SHBG output lowers the free fraction of every steroid SHBG carries, which feeds back on how much hormone reaches peripheral receptors. This subsection treats the carrier role.

Circulating E2 partitions across three pools. In a non‑pregnant adult only ~2–3% is unbound (“free”); ~38% is bound to SHBG and ~60% to albumin.C1 The free‑hormone hypothesis holds that only the unbound steroid crosses cell membranes to reach intracellular ER, so it is free — not total — E2 that sets receptor occupancy. A weaker variant, the bioavailable‑hormone model, counts free + albumin‑bound together, on the grounds that the albumin complex dissociates fast enough during capillary transit to surrender hormone to tissue; the SHBG‑bound pool is treated as sequestered. Which model is “correct” is tissue‑ and transit‑time‑dependent and still debated; both agree the SHBG‑bound fraction is the least available.C3

Albumin's affinity for E2 is three to four orders of magnitude below SHBG's, but its plasma concentration is ~1000× higher (Hammond 2016), so the two carriers end up holding comparable shares of E2.C2 Because neither protein is more than slightly occupied by the small steroid pool, the free fraction has a simple closed form:

Published SHBG–E2 K_d values span ~1.5–30 nM across assays. Structural/equilibrium work reports ~10–30 nM (Avvakumov 2010), whereas the operative constant embedded in clinical free‑hormone calculators (Vermeulen‑type, ~1.5 nM) is several‑fold tighter — partly because SHBG is a homodimer presenting two binding sites, and partly assay convention. The page uses ~20 nM as the molecular constant and a tighter ~2–5 nM operative value for free‑fraction arithmetic. The gap is not a temperature effect: SHBG–steroid affinity actually decreases (weakens) with rising temperature, so it is slightly lower at 37°C than in cold (4°C) assays for all three steroids (PMID 3702439) — the opposite direction from the structural-vs-operative gap.C2

Why this matters for trans HRT: SHBG suppresses free T more than free E2

SHBG does not bind all steroids equally. Its affinity ranks DHT (~1 nM) > testosterone (~3–5 nM) > estradiol (~20 nM) — androgens bind several‑fold tighter than E2.C1 In the free‑fraction equation, a tighter K_d means the [SHBG]/Kd term grows faster as SHBG rises, so the higher‑affinity ligand loses a larger proportion of its free fraction. Oral E2 raises SHBG ~67–171% (dose‑dependent; Ropponen et al. 2005);C1 that same SHBG rise suppresses free testosterone and free DHT proportionally more than it suppresses free E2 — a feature, not a bug, in transfeminine HRT, where the goal is to lower free androgen while maintaining free estrogen. It is also the mechanism behind the antiandrogenic reputation of oral (vs transdermal) estrogen and of EE‑containing combined oral contraceptives.

SHBG is unusually dynamic for a plasma carrier. Across healthy adults it spans ~20–100 nM (men lower, women higher); pregnancy — the largest sustained physiological estrogen exposure — raises maternal SHBG roughly 5–10× over the non‑pregnant level (Anderson 1976: ~5×, with later assays reporting up to ~10× at term).C2 The same study clocks the postpartum decline at a ~7‑day half‑life — an independent human cross‑check on the multi‑day SHBG turnover that sets the ~2‑week lag before plasma SHBG reaches a new steady state after any estrogen dose change.C2 Because [SHBG] enters the free‑fraction denominator directly, this several‑fold dynamic range is what makes route‑ and dose‑driven SHBG shifts clinically meaningful for free‑hormone exposure.

The clinical signature of this hepatic effect is its route dependence: the same molecule raises SHBG sharply by mouth and barely at all through skin, because only the oral route delivers the first‑pass portal concentration spike that saturates hepatic ER. Ropponen et al. 2005 measured exactly this contrast in a within‑subject crossover, and it is the single cleanest demonstration that route, not systemic E2 level, sets the hepatic readout.

4The E1S reservoir

Estrone‑3‑sulfate (E1S) is the most abundant circulating estrogen species in every hormonal state. It is metabolically inert at ER itself (sulfate group blocks C3 binding) but is reactivated to E1 in target tissues by STS, then reduced to E2 by HSD17B1. Its plasma half‑life of 10–12 h (Ruder 1972) makes it the rate‑limiting step for the apparent elimination of oral E2.C1

Postmenopausal endogenous E1 production is ~40 µg/day (Longcope 1986; Grodin 1973), not ~80 µg/day as earlier drafts had it — that higher number was likely a premenopausal early‑luteal value misapplied to postmenopause.C1

The reservoir explains the long apparent oral half‑life. Free E2 itself clears with IV t½ of 1–2 h. The 13–20 h apparent half‑life of oral E2 is the rate at which the E1S reservoir bleeds back into the free pool: ~320 µg‑equivalent of a 1 mg oral dose enters systemic E1S, then reactivates over 12–24 h via STS in target tissues.C2 Sublingual loads a smaller reservoir (only the swallowed fraction enters portal first‑pass), giving a shorter, sharper plasma curve. Transdermal builds essentially no oral‑style reservoir — the steady‑state E1S elevation comes from peripheral E2→E1→E1S conversion at standard cycling rates.

5VTE risk by route

The clinical end‑point of route‑dependent hepatic exposure is venous thromboembolism (VTE). Oral estrogen elevates risk roughly 4× baseline; transdermal does not.C1

The ESTHER 4.2× figure is from the original case‑control study (Canonico 2007, Circulation, PMID 17309934); pooled meta‑analyses dilute the signal because they include lower‑dose oral regimens and heterogeneous transdermal protocols. The 4.2 vs 0.9 contrast is the load‑bearing population‑level number behind modern guideline preference for transdermal over oral.C1

The cascade of attenuation (a heuristic)

Pregnancy plasma E2 is ~250× cycling baseline; pregnancy VTE risk is ~3–10×. The intuitive question is how the 250× plasma signal is buffered down to a 3–10× clinical signal. Earlier drafts presented a stepwise multiplicative chain (÷3 SHBG buffering × ÷30 SHBG induction saturation × ÷2 TFPI/protein S compensation × ×3 Virchow's triad) terminating at the empirical risk. The round‑2 fact‑check found multiple invented multipliers in this chain, and it is now framed strictly as a heuristic:

• SHBG buffering of free E2: earlier draft said ÷3; pregnancy SHBG rise drops the free fraction only to ~60% of nonpregnant baseline, so the actual buffering is closer to ÷1.7.C2
• SHBG induction saturation: the dominant attenuator (the dose‑response curve plateaus near pregnancy levels), but the specific ÷30 was modeled, not measured.
• TFPI ↑ / protein S ↑: wrong direction. In pregnancy, free protein S falls to ~38% of nonpregnant (Comp 1986), and TFPI is lower than nonpregnant (~7.1 vs 8.2 ng/mL).C1C1 There is no net anticoagulant compensation; this arm is also procoagulant.
• ×3 Virchow's triad: invented multiplier. Virchow's triad is qualitative; the empirical 3–10× comes from cohort studies (Heit 2005, Pomp 2008, Sultan 2013), not a derivable mechanistic chain.C2

The qualitative claim — plasma E2 ≠ free E2 ≠ hepatic effect ≠ net coagulation ≠ VTE incidence — is fully supported. The specific multipliers are not. Read the chain as a teaching device for the structure of attenuation, not as a sourced calculation.

6Ethinyl estradiol

Ethinyl estradiol (EE) is 17α‑ethinyl‑17β‑estradiol. The single 17α‑ethinyl substitution makes C17 a quaternary carbon — bearing the 17β‑OH, the 17α‑ethinyl (−C≡CH), and the two ring carbons C13/C16 — with no carbinol hydrogen for HSD17B2 to abstract.C1 The 17β‑OH is still there. An earlier draft of this synthesis incorrectly stated that EE has “no C17‑OH” — the round‑2 codex fact‑check caught this. The chemistry is right (HSD17B2 cannot oxidize EE), the prior wording was wrong.C1

The mechanistic consequences:

• EE persists as parent drug. Without HSD17B2 oxidation, the dominant inactivation arm for E2 (E2 → E1 → E1S in the liver) is gone for EE. Clearance proceeds only via sulfation (SULT1E1 forms EE‑3‑sulfate; reversible via STS), glucuronidation (UGT1A1/1A3/1A9, plus UGT2B7 at C17), and CYP3A4/2C9 hydroxylation. All slower than the HSD17B2 arm for E2.
• Gut wall does ~44% of EE first‑pass conjugation. Back & Rogers 1982 cannulated the human portal vein and measured this directly — the single load‑bearing primary measurement for “gut wall is a major first‑pass site,” made specifically on EE.C1 Back 1990 review notes the gut‑wall product is principally sulfate (likely SULT1E1‑dominated, despite SULT1A1 being more abundant, because SULT1E1 wins at physiological substrate concentrations on K_m — the earlier draft had this backwards).C2
• Hepatic extraction of free EE is ~25% on first pass (Back & Rogers 1982), not ~20% as earlier drafts had it.C1 Compare with ~90% extraction for E2 — EE escapes most of the hepatic first‑pass that destroys oral E2.
• Single‑dose oral EE C_max is ~70 pg/mL for 30 µg (extrapolated linearly from Brody/Turkes/Goldzieher 1989 70 µg data, mean C_max ~160 pg/mL).C1 The frequently cited ~95 pg/mL is the steady‑state C_max (Yasmin label; 1.5–2× accumulation factor); earlier drafts mislabeled this as single‑dose.
• EE half‑life is 18–24 h at steady state.C1
• EE does not meaningfully bind SHBG (relative affinity ~0.18 vs E2 ~9, i.e. ~50× weaker).C1 When EE drives SHBG up via hepatic ER, the rising SHBG does not trap EE itself but crashes the free fraction of endogenous E2 — the mechanism by which combined OCs maintain biological activity while suppressing ovarian E2 to near‑castrate.
• Per‑weight hepatic potency vs oral E2 is large but endpoint‑specific: 75–1000× on the various hepatic proteins per Kuhl 2005's weight‑basis table.C1 The “~100× per mole” shorthand is a reasonable average over the SHBG endpoint specifically; it is NOT a universal constant across hepatic outputs. At the peripheral ER, EE is only ~1.5× as potent as E2 per mole.

Brody, Turkes & Goldzieher 1989, Contraception 40:269 (reproduced by Goldzieher 1990). Single‑dose 2‑tablet (= 70 µg EE) administration to n=24 women. Mean C_max ~160 pg/mL at 1 h; highest‑responder peak ~300 pg/mL — 5× the lowest responder. The 4–6 h plateau is enterohepatic recirculation. The 5× spread at n=24 with the same drug and dose is the inter‑individual variability the model curves do not show.

SHBG response to combined oral contraceptives (Stegeman 2013)

Source: Stegeman 2013, J Thromb Haemost.C1 Androgenic progestins (LNG) blunt SHBG rise; antiandrogenic ones (drospirenone, cyproterone) amplify it. Diane‑35 reaches roughly pregnancy‑level SHBG. The estrogen choice (EE vs E2‑valerate) matters at least as much as the progestin choice — Qlaira (E2V + dienogest) sits at +50–80% SHBG and ~4–6/10,000 woman‑years VTE, vs Yasmin's +200–300% SHBG and ~10–12/10,000 woman‑years.

7Prodrug strategy: EC508, E2MATE

The unsolved formulation problem: design an oral E2 prodrug that delivers physiological systemic E2 levels without hepatic ER‑α saturation. The chemical approach attempted twice: install a sulfamoyl group (−SO₂NH₂) that tightly ligates the active‑site Zn²⁺ of carbonic anhydrase II (CAII) inside red blood cells. RBC sequestration carries the drug past the liver in the cellular compartment rather than as free drug in plasma; plasma esterases then slowly release E2 in systemic circulation.

E2MATE

E2MATE (estradiol‑3‑sulfamate) installed the sulfamoyl group on the C3 phenolic oxygen. This protected C3 from SULT/UGT and bound CAII, but the C3‑O‑SO₂NH₂ bond is also recognized by STS. Instead of being cleanly cleaved to release E2, the sulfamate covalently modifies STS's active‑site formylglycine residue via mechanism‑based sulfamoyl transfer.C1 The FGly‑N‑sulfate adduct is stable; STS is permanently inactivated; both the prodrug's own activation and the body's normal E1S reactivation cycle are abolished. This is a mechanism‑based irreversible inhibitor, not a transition‑state mimic — earlier drafts of this synthesis described it as a TSI, which is the wrong mechanistic class (round‑2 mechanism fact‑check finding 1).C1 E2MATE has been repurposed as an STS inhibitor for endometriosis.

EC508

EC508 = estradiol‑17β‑(1‑(4‑(aminosulfonyl)benzoyl)‑L‑proline). The redesign moves the sulfamoyl group off the C3 oxygen (where STS attacks) and onto a separate L‑proline‑based handle attached at C17 via an esterase‑cleavable ester linker.C2 Estradiol is intact; the sulfamoyl head ligates CAII inside RBCs; plasma esterases slowly cleave the C17 ester bond to release free E2 plus a prolyl‑aryl‑sulfonamide fragment.

Development status: EC508 was developed by Evestra Pharmaceuticals. As of 2021, the transfemscience.org EC508 article notes “No Further Development.” It never progressed to human clinical trials; small‑pharmaceutical funding constraints are the most plausible explanation. The chemistry works in principle and in animals.C2

8Inter‑individual variability

Real PK variability is much larger than central‑estimate tables suggest. The Goldzieher 1990 reproduction of Brody/Turkes/Goldzieher 1989 shows a 5× spread in single‑dose oral EE C_max across n=24 women receiving identical 70 µg doses.C1 Stanczyk 2013 reviews comparable spreads across oral and transdermal E2.C1 Drivers of variability:

• UGT2B7 and SULT1E1 polymorphisms — affect C17 and C3 conjugation rates; can move oral F by 2–3×.
• UGT1A1*28 (Gilbert's syndrome variant) — reduces hepatic glucuronidation; modestly raises E2 AUC.
• Body composition — adipose tissue expresses aromatase; obese individuals have higher peripheral testosterone→E2 conversion. BMI affects E1S apparent V_d.
• Age — UGT activity decreases ~30% in older adults; gut motility and microbiome change.
• Ethnicity — CYP1A2 and CYP3A4 inducibility varies between populations; affects catechol pathway contribution and EE clearance.
• Gut microbiome composition — controls bacterial β‑glucuronidase activity, hence enterohepatic recirculation magnitude.

Bimodal oral EE bioavailability has been reported in some studies, with a high‑F subpopulation distinct from a low‑F majority.C2 Clinical implication: literature central‑estimate doses are starting points, not endpoints. Real PK has CV ~30–60% on most parameters and follow‑up bloodwork is essential for any HRT regimen targeting a specific E2 range.

9Steady‑state vs single‑dose

Single‑dose PK and steady‑state PK differ in three ways relevant to HRT.

Accumulation. For a drug with apparent t½ longer than the dosing interval, plasma concentrations accumulate to steady‑state values of (single‑dose) × (accumulation factor). For oral E2 (apparent t½ ~15 h, q24h dosing) the accumulation factor is ~1.4×. For EE (t½ ~20 h, q24h) it is 1.5–2× (Yasmin label: single‑dose 30 µg C_max ~70 pg/mL → steady‑state ~95 pg/mL).C1

E1S reservoir loading. The reservoir takes ~3–5 days (3–5× E1S t½ of 10–12 h) to reach steady state. Until then, plasma E2 trough is lower than it will be at chronic dosing — new oral HRT patients commonly under‑estimate their eventual steady‑state level from day‑1 measurements.

SHBG transcriptional time constants. SHBG protein turnover sets a slow time constant of ~4 days; chronic dose changes don't show in plasma SHBG for ~2 weeks. Clotting factor changes are faster (~3–7 days) but still substantially slower than the PK time constants. Single‑dose hepatic effects are essentially undetectable; only chronic dosing produces the protein‑synthesis shifts that drive VTE risk.

10Trans HRT specifics

Practical estrogen choices in transfeminine HRT, with the synthesized pharmacology applied to clinical decisions.

Target serum E2: 150–250 pg/mL (mid‑cycling premenopausal female range) for full feminization and testosterone suppression. The 100 pg/mL lower bound is commonly cited as the threshold for adequate gonadotropin suppression in monotherapy.

Anti‑androgen co‑medication: spironolactone, cyproterone acetate (not US‑available), and bicalutamide are the three commonly used agents; GnRH analogs (leuprolide, triptorelin) are the most effective and most expensive option. Monotherapy with high‑dose E2 (especially injectable) can suppress LH/FSH and thereby testosterone production without an anti‑androgen — the “estrogen monotherapy” protocols, increasingly popular, exploit this.

EE is contraindicated in modern trans HRT (Endocrine Society 2017, WPATH SOC8). CEE (Premarin) is similarly out of favor for the same hepatic‑disproportion reason.

Route selection in patients with VTE risk factors (factor V Leiden, prior DVT, smoking, obesity, age >50) should default to transdermal or IM‑ester depot rather than oral. The 4× OR contrast between oral and transdermal is the clinical reason to make this choice the default rather than the exception.

11Confidence index

The C1–C5 inline tags throughout this page each link to the specific entry in evidence.html that supports (or doesn't support) the claim. A non‑exhaustive map of the load‑bearing claims by tier:

The C1–C5 system is intentionally generous — every numerical or mechanistic claim above carries a tag. The pattern: basic enzyme biochemistry and population‑level PK numbers are stable C1; model‑calibrated quantities and inferred mechanisms degrade to C2–C3; preliminary clinical signals and unmeasured chemistry are C4–C5.

What round‑2 fact‑checks moved (since the earlier narrative)

Listed compactly; full discussion in evidence.html §11.

• SHBG EC50 1500 pg/mL — re‑framed as v4 calibration target, not measured. Selva & Hammond 2009 misattribution removed; Kalme 1999 is the right HepG2 anchor (at 0.5–2.5 µM).C1
• EE has a 17β‑OH — earlier “no C17‑OH” wording corrected. C17 is quaternary; HSD17B2 fails because there is no carbinol H to abstract, not because the alcohol is missing.C1
• E2MATE inhibits STS by covalent modification of formylglycine, not by transition‑state mimicry.C1
• Cascade buffering ÷3 → ÷1.7; TFPI in pregnancy decreases (procoagulant), not increases; ×3 Virchow's triad is invented.C1C2
• EE single‑dose C_max ~70 pg/mL for 30 µg; earlier ~95 was steady‑state mislabeled as single‑dose.C1
• EE hepatic extraction ~25% (Back & Rogers 1982), not ~20%.C1
• Sublingual direct F is uncertain; v4 uses 5%, older textbooks 20–25%, marmoset 10%, human absolute not cleanly measured.C2
• Gut SULT1E1 dominates over SULT1A1 at physiological [E2] (K_m argument, not abundance); earlier draft had this backwards.C2
• SULT1E1 lowest K_m “for estrogens”, not “for any substrate.”C2
• AUC values in narrative time‑course curves are model integrals, not published anchors. Doll 2022 only directly reports AUC(0–8) and a 1.8× SL/oral ratio.
• E2 mass balance fractions are v4 calibration outputs, not radiolabel data.C3

12See also

• evidence.html — per‑claim primary‑source quotes, figures, and confidence tags (64 entries)
• reference.html — glossary, enzyme list, PK parameter definitions, papers (~110 anchors)
• synthesis‑narrative.html — the long‑form narrative version (156 KB), preserved as archive with full v4 model figures and complete pre‑round‑2 fact‑check discussion
• Interactive v4 PK viewer — pick a scenario, scrub time, watch concentrations and fluxes update
• transfemscience.org — literature‑reviewed PK aggregations for trans HRT
• Wikipedia: Pharmacokinetics of estradiol; Wikipedia: Ethinylestradiol — well‑referenced indices to primary literature
• Aly's hormones — community PK references for IM (paper‑aly‑im) and SL (paper‑aly‑sl) routes