Estradiol: the pathway in the body · Biomarker & Laboratory Analysis

This page shows the biochemical pathway behind the laboratory value Estradiol (E2): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Estradiol is the main oestrogen, a steroid hormone made from cholesterol. Aromatase forms it from testosterone; in the cell it binds the oestrogen receptor, which co-determines which genes are read. Through feedback it helps regulate LH and FSH.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Cholesterol → Pregnenolone CYP11A1 · NADPH, Oxygen The enzyme CYP11A1 shortens the side chain of cholesterol. It needs oxygen and electrons that come from NADPH. All steroid pathways branch off from pregnenolone. Source 1↑ supplies Pregnenolone is the common precursor of all steroid hormones. From here the pathways to cortisol, aldosterone and the sex hormones branch off; which one prevails depends on the enzymes in each tissue. established physiology Source 1
   ⚖ When the balance tips

   too much — If pregnenolone builds up, more of it enters the pathways whose enzymes are abundant in the tissue; the path to the sex hormones absorbs as much as CYP17A1 converts.

   too little — If little pregnenolone is available, all steroid pathways share the scarce supply, and DHEA and the estrogens also form more slowly.

   established physiology · Source 1

2. Pregnenolone → DHEA CYP17A1 · NADPH, Cytochrome b5 CYP17A1 first attaches an OH group and then splits off two carbon atoms. This forms DHEA, a precursor of the sex hormones. Tissues convert it to androgens or estrogens as needed. Source 1↑ supplies DHEA is a weak androgen and above all raw material: in tissues it is converted to androstenedione and on to testosterone or estrogens. The adrenal gland releases it in large amounts, mostly as DHEA sulfate. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of DHEA arrives, more androstenedione forms in the tissue; depending on the enzymes present, the excess goes into androgens or, via aromatase, into estrogens.

   too little — If little DHEA is available, less androstenedione forms in the tissue, and the supply for androgens and estrogens from this source becomes scarcer.

   established physiology · Source 1

3. DHEA → Androstenedione 3β-HSD · NAD⁺ 3β-hydroxysteroid dehydrogenase converts DHEA, shifting a double bond in the ring system. In adipose tissue, aromatase turns it directly into estrone. Source 1↑ supplies Androstenedione is a junction: 17β-HSD turns it into testosterone, aromatase directly into estrone. In adipose tissue a large part of estrogen formation runs this way, especially once the ovaries no longer form estrogens. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If a lot of androstenedione reaches adipose tissue, aromatase converts more of it to estrone; how much depends on the amount of aromatase in the tissue.

   too little — If little androstenedione is available, aromatase outside the ovaries has hardly any starting material, and less estrone forms there.

   established physiology · Source 2

4. Androstenedione → Testosterone 17β-HSD · NADPH A 17β-hydroxysteroid dehydrogenase turns androstenedione into testosterone. Both substances are the direct precursors of the estrogens. Testosterone also acts as an androgen itself. Source 1↑ supplies Testosterone acts at the androgen receptor itself and is also a starting material: aromatase turns it into estradiol. How much testosterone becomes estrogen this way depends on the aromatase in each tissue. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If a lot of testosterone arrives, aromatase converts a larger share of it into estradiol, as far as the enzyme is present in the tissue.

   too little — If little testosterone is available, aromatase also forms less estradiol; this affects tissues that make their estrogen locally, such as the brain and adipose tissue.

   established physiology · Source 2

5. Testosterone → Estradiol Aromatase (CYP19A1) · NADPH, Oxygen Aromatase (CYP19A1) removes one carbon atom and turns the first ring into an aromatic ring. Testosterone thus becomes estradiol, and androstenedione becomes estrone. In the ovaries, FSH controls this step. Source 2, 1↑ supplies Aromatase is the only step that forms estrogens. FSH controls it in the granulosa cells of the ovary; in adipose tissue and the brain, cells form estradiol locally, where it acts right away. established physiology Source 2
   ⚖ When the balance tips

   too much — If aromatase forms a lot of estradiol, it signals back to the hypothalamus and pituitary through feedback; as a rule, less LH and FSH are then released, and the supply falls.

   too little — If aromatase is slowed or little precursor is available, less estradiol forms; the pituitary then releases more LH and FSH, and androgens build up before this step.

   established physiology · Source 4, 2

6. Estradiol in the blood → Estrogen receptor Inside the cell, estradiol binds to an estrogen receptor. Two receptors join to form a pair and bind to specific sections of DNA. ERα and ERβ are distributed differently across tissues. Source 4↑ supplies ERα and ERβ are distributed differently across tissues: ERα predominates in the uterus and liver, for example, ERβ in the ovary among others. The two partly act in opposite directions; their ratio shapes the cell's response. observed in studies Source 4
   ⚖ When the balance tips

   too much — If a lot of estradiol is present, more receptors are occupied; after prolonged binding the cell breaks down part of the receptors, and the response becomes weaker.

   too little — If little estradiol is present, the receptors mostly remain unoccupied; in cell experiments, growth factors can then partly switch them on even without hormone.

   observed in studies · Source 4

7. Estrogen receptor → Reading genes Together with other proteins, the receptor helps determine how strongly individual genes are read. Which genes these are depends on the tissue. In this way estradiol builds up the uterine lining and slows bone resorption, for example. Source 4↑ supplies Through the genes read, estradiol builds up the lining of the uterus, slows bone resorption by osteoclasts and controls the release of LH and FSH in the hypothalamus and pituitary. established physiology Source 4
   ⚖ When the balance tips

   too much — If a lot of estradiol acts for a long time, the uterine lining keeps building up and more progesterone receptor is made, a classic target gene of the estrogen receptor.

   too little — If little estradiol is present, the lining is hardly built up, and in bone resorption by osteoclasts outweighs formation; bone remodelling shifts.

   established physiology · Source 4

8. Estradiol in the blood → Estrone 17β-HSD type 2 · NAD⁺ A 17β-hydroxysteroid dehydrogenase converts estradiol into estrone. The reaction runs in both directions; the tissue determines which one prevails. As estrone sulfate it serves as a reserve in the blood. Source 1, 5↕ both, depending on amount Estrone acts much more weakly at the receptor. As estrone sulfate it circulates in the blood for a long time and serves as a reserve: steroid sulfatase removes the sulfate, and a 17β-HSD turns it back into estradiol in the tissue. established physiology Source 5
   ⚖ When the balance tips

   too much — If conversion to estrone prevails in a tissue, less estradiol acts at the receptor there; part of it can later be recovered via 17β-HSD type 1.

   too little — If little estrone is formed, estradiol stays in its active form longer, and the reserve of estrone sulfate in the blood becomes smaller.

   established physiology · Source 5

9. Estrone → 2-Hydroxyestrone CYP1A1, CYP1B1 · NADPH Enzymes of the cytochrome P450 family, mainly CYP1A1 and CYP1B1, attach an OH group to the aromatic ring. The products barely bind to the receptor any more. Source 5↕ both, depending on amount 2-hydroxylation is a main route of breakdown; the products barely bind to the receptor any more. Catechol estrogens can, however, be oxidised to reactive quinones, which bind to DNA in cell experiments. observed in studies Source 5
   ⚖ When the balance tips

   too much — If 2-hydroxyestrone builds up because COMT cannot keep pace, more of it is oxidised to quinones; in cell experiments these bind to DNA.

   too little — If little 2-hydroxyestrone is formed, more estrone remains for other routes, such as 4- or 16α-hydroxylation, or it turns back into estradiol.

   observed in studies · Source 5

10. 2-Hydroxyestrone → 2-Methoxyestrone COMT · SAM, Magnesium Catechol-O-methyltransferase (COMT) attaches a methyl group. It comes from SAM, the methyl group donor of the cell. This makes the catechol estrogens more stable. Source 5↓ depletes Methylation by COMT makes catechol estrogens stable, so that they are no longer oxidised to reactive quinones. For this it uses up SAM, the methyl group donor from methionine metabolism. observed in studies Source 5
    ⚖ When the balance tips

    too much — If many catechol estrogens arrive, COMT uses more SAM; the methyl groups come from the same pool the cell needs for other methylations.

    too little — If little SAM or magnesium is available, COMT methylates more slowly; catechol estrogens persist longer and can be oxidised to quinones.

    observed in studies · Source 5

11. 2-Methoxyestrone → Conjugates UGT, SULT Transferases couple the breakdown products to glucuronic acid or sulfate. Now water-soluble, they leave the body via urine and bile. Sulfatases in tissues can split the sulfates again. Source 5↓ depletes Coupled to glucuronic acid or sulfate, estrogens lose their action at the receptor and become water-soluble. Sulfates remain as a reserve in the blood; sulfatases in tissues can split them back into active estrogens. established physiology Source 5
    ⚖ When the balance tips

    too much — If many conjugates are formed, more estrogens leave the body via urine and bile, and fewer are available to the receptors.

    too little — If coupling runs slowly, free estrogens and catechol estrogens remain longer in the tissue and in the blood.

    established physiology · Source 5

Further stations

• Cholesterol — source of all steroids All steroid hormones derive from cholesterol. For this, it is channelled into the mitochondria in the ovary, testis, adrenal cortex and adipose tissue. LH from the pituitary speeds up this transport. Source 1↑ supplies Cholesterol provides the ring framework of all steroid hormones. Transport into the mitochondria is the slowest step; LH from the pituitary speeds it up in the ovary and testis and so sets how much supply arrives. established physiology Source 1
  ⚖ When the balance tips

  too much — If more cholesterol enters the cell than is converted, the cell stores it as esters in lipid droplets; how much hormone forms depends on transport into the mitochondria, not on the store.

  too little — If little cholesterol reaches the mitochondria, less pregnenolone forms, and all downstream steroid hormones are replenished more slowly.

  established physiology · Source 1

• Estradiol in the blood — bound to SHBG In the blood, most estradiol is bound to SHBG and to albumin. Only the unbound fraction passes into the cells. The amount of SHBG determines how much is free. Source 3↕ both, depending on amount SHBG is both store and brake: bound estradiol escapes rapid breakdown but does not act. Only the free fraction reaches the receptors; the amount of SHBG determines how much is free. established physiology Source 3
  ⚖ When the balance tips

  too much — If there is a lot of SHBG in the blood, it binds more estradiol; the free fraction that enters the cells becomes smaller, even if the total amount stays the same.

  too little — If little SHBG is present, more estradiol is free; it enters the cells more easily but is also broken down faster.

  established physiology · Source 3

Cofactors in this pathway

• NADPH — Electron donor for CYP11A1, CYP17A1 and aromatase; without NADPH each of these steps stalls Source 1, 2
• Iron — Central atom of the haem in the cytochrome P450 enzymes involved; there it binds the oxygen Source 1In the ORY catalogue as a laboratory value: Eisen
• NAD⁺ — Accepts hydrogen at 3β-HSD and at 17β-HSD type 2; 17β-HSD type 2 thus turns estradiol into estrone Source 1In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• SAM (from methionine) — Methyl group donor of COMT when catechol oestrogens are converted; with little SAM, COMT methylates more slowly Source 5In the ORY catalogue as a laboratory value: Methionin
• Magnesium — Ion in the active site of COMT, which methylates catechol oestrogens; without magnesium it works more slowly Source 5In the ORY catalogue as a laboratory value: Magnesium
• Zinc — Keeps the two zinc fingers in the DNA-binding part of the oestrogen receptor in shape; only then does it bind DNA Source 7In the ORY catalogue as a laboratory value: Zink

What acts on this pathway

• Aromatase inhibitors — Anastrozole attaches to aromatase and blocks it; as a result, the body forms less estradiol. The earlier steps of the pathway are not affected. Source 6

Sources

1. Miller WL, Auchus RJ. The molecular biology, biochemistry, and physiology of human steroidogenesis and its disorders. Endocr Rev 2011 · PubMed 21051590
2. Simpson ER, Mahendroo MS, Means GD et al. Aromatase cytochrome P450, the enzyme responsible for estrogen biosynthesis. Endocr Rev 1994 · PubMed 8076586
3. Hammond GL. Plasma steroid-binding proteins: primary gatekeepers of steroid hormone action. J Endocrinol 2016 · PubMed 27113851
4. Heldring N, Pike A, Andersson S et al. Estrogen receptors: how do they signal and what are their targets. Physiol Rev 2007 · PubMed 17615392
5. Thomas MP, Potter BV. The structural biology of estrogen metabolism. J Steroid Biochem Mol Biol 2013 · PubMed 23291110
6. Cuzick J. Anastrozole. Drugs Today (Barc) 2005 · PubMed 16034487
7. Arao Y, Korach KS. The physiological role of estrogen receptor functional domains. Essays Biochem 2021 · PubMed 34028522

Related pathways

• DHEA — cholesterol
• Testosterone — cholesterol
• Cholesterol and lipoproteins — cholesterol
• Oestrogen breakdown — estrone
• Cortisol — cholesterol

As of 2026-09-16. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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