Lecture 5: Sex Steroid Synthesis & Action

Steroid hormones are classified by carbon count: C21 = progestins/corticoids, C19 = androgens (DHEA, androstenedione, testosterone, DHT), C18 = estrogens (estrone, estradiol, estriol). The only enzyme that crosses between classes is aromatase (CYP19A1): it aromatizes the A-ring and removes the C19 methyl, converting a C19 androgen into a C18 estrogen (androstenedione → estrone; testosterone → estradiol; 16α-OH-DHEA via fetal-placental unit → estriol). Cortisol is C21 not C19, 5α-reductase converts testosterone to the still-C19 DHT (no class change), and 3β-HSD interconverts progestins/corticoids (still C21). LO 1 is about correctly classifying hormones by structure and recognizing the transitions between classes.

Aromatase performs the C19 → C18 transition (androgen → estrogen) by aromatizing the A-ring and removing the angular C19 methyl. Aromatase inhibitors (letrozole, anastrozole, exemestane) thus prevent estrogen synthesis and cause modest accumulation of the C19 substrates upstream (androstenedione, testosterone). Contrast with the other class transitions: desmolase/P450scc cleaves the C22–C27 side chain (C27 → C21, shared for all steroids), and 17,20-lyase cleaves the C20–C21 side chain of 17-OH-progesterone/pregnenolone (C21 → C19, commits toward androgens). Knowing which enzyme changes the carbon-count class is the key LO 1 pattern.

DHEA-S and DHEA have negligible direct androgen receptor activity. Their androgenicity requires peripheral conversion: DHEA-S → DHEA (sulfatase) → androstenedione (3β-HSD) → testosterone (17β-HSD) → DHT (5α-reductase). This conversion occurs in skin, hair follicles, and adipose tissue. DHEA-S is sulfated by DHEA-sulfotransferase (SULT2A1) in the adrenal → long half-life (hours to days) → useful androgen reservoir. Elevated in adrenarche, CAH, PCOS, adrenal adenomas. High DHEA-S preferentially suggests an adrenal (not ovarian) androgen source.

DHEAS is produced exclusively by the adrenal gland (zona reticularis). Markedly elevated DHEAS (>700 µg/dL) strongly suggests adrenal etiology: adrenocortical carcinoma (most common cause of very high DHEAS), adrenal adenoma, or rarely ectopic ACTH. Normal or mildly elevated DHEAS is seen in PCOS (ovarian source). Ovarian androgens cause elevated testosterone/androstenedione but not isolated DHEAS elevation.

Aromatase (CYP19A1) converts androgens → estrogens: testosterone → estradiol; androstenedione → estrone. An aromatase-secreting adrenal adenoma or testicular Sertoli cell tumor elevates estradiol → suppresses LH/FSH via negative feedback → low testosterone → gynecomastia and infertility. Aromatase is the only enzyme capable of converting C19 androgens to C18 estrogens (requires three oxidation steps). Treatment: aromatase inhibitors (anastrozole, letrozole) for tumor-associated aromatase excess or surgical resection.

In an ovulatory cycle, corpus luteum progesterone peaks around day 21 (mid-luteal phase). A value >3 ng/mL suggests ovulation occurred; >10 ng/mL reflects a robust luteal phase. A value of 2.1 ng/mL is below threshold → suggests anovulation or inadequate luteal function. If anovulatory: evaluate for PCOS (LH:FSH ratio, free testosterone, SHBG), thyroid dysfunction (TSH), hyperprolactinemia (prolactin), and hypothalamic amenorrhea (FSH, LH, estradiol). Note: the progesterone surge is also used in temperature charting (rises ~0.5°F post-ovulation) and home LH testing confirms the ovulatory trigger.

StAR (Steroidogenic Acute Regulatory) protein transports cholesterol from outer to inner mitochondrial membrane — the rate-limiting step for ALL steroid synthesis. Its deficiency causes congenital lipoid adrenal hyperplasia: no cortisol, no aldosterone, no sex steroids. Salt-wasting crisis from aldosterone deficiency + female phenotype in 46,XY from absent testosterone/DHT. Cholesterol accumulates in the adrenal and gonads (lipoid appearance). Aromatase deficiency prevents estrogen synthesis but not all steroids. CYP17A1 deficiency blocks sex steroids + cortisol but spares mineralocorticoids (↑BP instead of salt-wasting).

CYP17A1 deficiency blocks: (1) 17α-hydroxylation → no cortisol, (2) 17,20-lyase activity → no androgens or estrogens. Precursors shunt to mineralocorticoids (↑DOC, ↑corticosterone) → hypertension + hypokalemia. In 46,XY: no testosterone → no Wolffian duct development, no DHT → female external genitalia; but AMH intact (from Sertoli cells) → no Müllerian structures → no uterus. The key distinguishing feature from 21-hydroxylase deficiency: hypertension (not salt-wasting) and NO androgen excess.

The two-cell model: LH → theca cells → androstenedione/testosterone → diffuses to granulosa cells; FSH → granulosa cells → upregulates aromatase (CYP19A1) → converts androgens → estradiol. This is the basis of estradiol production in the follicular phase. PCOS disrupts this: ↑LH/FSH ratio → excess theca androgen production without adequate aromatase activity → hyperandrogenism + anovulation.

Finasteride specifically inhibits type 2 5α-reductase (predominant in prostate and genital skin). Dutasteride inhibits both type 1 (skin, liver, other tissues) AND type 2 — resulting in more complete DHT suppression (>90% vs ~70% with finasteride). This more potent DHT suppression can translate to greater prostate volume reduction. Both drugs increase testosterone (less converted to DHT) and may cause gynecomastia through increased aromatization. Note: neither is an androgen receptor antagonist (unlike flutamide, bicalutamide, enzalutamide).

Estriol production requires the fetoplacental unit: fetal adrenal → DHEA-S → fetal liver 16α-hydroxylates → 16-OH-DHEA-S → placental CYP19A1 converts to estriol. Placental aromatase deficiency: estriol undetectable + androgens cannot be converted → maternal virilization (clitoromegaly, acne, deepened voice) and female fetal virilization. Note: Down syndrome (trisomy 21) also shows LOW uE3 on quad screen (due to impaired fetal adrenal/liver maturation), but is not zero and does not cause maternal virilization. Quad screen for Down: ↓AFP, ↓uE3, ↑hCG, ↑inhibin A.

Classic presentation of salt-wasting congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency. The enzyme block: ↑17-OH-progesterone (substrate accumulation → diagnostic marker), ↓cortisol → ↑ACTH → shunts precursors to androgens (virilization of 46,XX) + ↓aldosterone → salt-wasting crisis. The elevated androgens in utero cause virilization of XX external genitalia. 11β-hydroxylase deficiency also causes CAH with virilization but causes hypertension (not salt-wasting) from ↑DOC. 17α-hydroxylase causes hypertension, no virilization.

5α-reductase type 2 deficiency: testosterone is produced normally by Leydig cells but cannot be converted to DHT in target tissues. At puberty: LH surge → high testosterone → virilization via testosterone's direct androgen receptor activity (AR works; it's the 5α-reductase that's broken). Labs: ↑testosterone, ↓DHT, ↑T:DHT ratio (>30:1 diagnostic). LH may be slightly elevated (partial AR resistance feedback). In CAIS, the AR itself is broken → no virilization at puberty + very high LH (complete AR resistance). The distinguishing feature: 5α-reductase deficiency VIRILIZES at puberty; CAIS does NOT.

21-Hydroxylase (CYP21A2) is required for: (1) 17-OHP → 11-deoxycortisol → cortisol; (2) Progesterone → deoxycorticosterone → aldosterone. Without CYP21A2: 17-OHP accumulates (pathognomonic marker; detected on newborn screening); ACTH rises (absent cortisol feedback) → substrate shunted into androgen pathway (→ androstenedione → testosterone → DHT) → virilization of 46,XX fetus. Absent aldosterone → salt-wasting (hyponatremia, hyperkalemia, hypotension, dehydration). Compare: 11β-hydroxylase deficiency also causes virilization + elevated 17-OHP (mildly) but causes hypertension (DOC accumulates → mineralocorticoid activity) rather than salt-wasting.

Insulin resistance and hyperinsulinemia suppress hepatic SHBG synthesis → low SHBG → greater proportion of testosterone is free/bioavailable → hyperandrogenism even when total testosterone is only borderline elevated. This is the mechanism in PCOS: insulin resistance → ↓SHBG → ↑free T → hirsutism, acne, anovulation. Weight loss and metformin (improve insulin sensitivity) → ↑SHBG → ↓free T → improvement in symptoms.

SHBG is produced by the liver; insulin and IGF-1 suppress its synthesis. Hyperinsulinemia in PCOS → ↓SHBG → larger free testosterone fraction despite normal total testosterone. The Free Androgen Index (FAI = total T/SHBG × 100) reflects this. Clinical pearl: OCPs (estrogen component) raise SHBG → ↓free androgens → treat hirsutism/acne in PCOS. Exogenous androgens also suppress SHBG. Thyroid hormone elevates SHBG (explains elevated total T4 in pregnancy without clinical hyperthyroidism).

Anastrozole (and letrozole, exemestane) are aromatase inhibitors (AIs) that block CYP19A1 → prevent peripheral conversion of androgens → estrogen. They are preferred over tamoxifen as adjuvant therapy in postmenopausal ER+ breast cancer (ATAC trial: superior disease-free survival). Key point: AIs only work in postmenopausal women (not premenopausal) because ovarian estrogen production overwhelms peripheral block. Postmenopausal women rely primarily on peripheral aromatization. Fulvestrant is a selective estrogen receptor degrader (SERD). Tamoxifen is a SERM, not an AI.

Tamoxifen is a SERM with tissue-selective activity: ER antagonist in breast (blocks tumor proliferation) but ER partial agonist in uterus/endometrium. This endometrial agonism leads to proliferation → endometrial hyperplasia → up to 3× increased risk of endometrial carcinoma. Any postmenopausal woman on tamoxifen with vaginal bleeding requires urgent endometrial biopsy. Raloxifene is preferred for breast cancer prevention in postmenopausal women with intact uterus as it lacks this agonist effect in the endometrium.

Clomiphene is an ER antagonist at the hypothalamus. By blocking estrogen receptors in the hypothalamus, it prevents estrogen from exerting negative feedback on GnRH neurons → GnRH pulse frequency increases → ↑LH and FSH from pituitary → follicle recruitment and growth → ovulation. It is first-line for ovulation induction in anovulatory infertility (PCOS). Side effects: hot flashes (anti-estrogenic), multiple gestation (multiple follicle recruitment), ovarian hyperstimulation syndrome (rare), hostile cervical mucus (can reduce conception despite ovulation).

In postmenopausal women, peripheral aromatization (adipose, liver, muscle) is the only estrogen source → anastrozole fully suppresses estrogen. In premenopausal women, blocking peripheral aromatase lowers estrogen → ↑FSH → stimulates ovarian follicles → granulosa cell aromatase (abundant) produces compensatory estradiol. Solution: combine aromatase inhibitor with ovarian suppression (GnRH agonist) in premenopausal ER+ breast cancer. SOFT and TEXT trials established GnRH agonist + AI as superior to tamoxifen alone in high-risk premenopausal patients.

Clomiphene is a SERM: ER antagonist at hypothalamus and pituitary → blocks estrogen negative feedback → pituitary interprets low estrogen → ↑GnRH → ↑FSH/LH → follicle recruitment and ovulation. Clomiphene is ER agonist at bone and partially at endometrium (thickens cervical mucus — can reduce fertility at high doses). Key comparison: Letrozole (aromatase inhibitor) reduces estrogen synthesis → same net effect (↑FSH/LH) but fewer anti-estrogenic endometrial effects → now preferred first-line for PCOS anovulation (Legro et al., NEJM 2014: higher ovulation and live birth rates vs. clomiphene).

Mifepristone (RU-486) is a progesterone receptor (PR) antagonist. By blocking PR in the endometrium and myometrium, it: (1) causes decidual breakdown → endometrial detachment; (2) increases uterine sensitivity to prostaglandins and oxytocin; (3) promotes cervical ripening. It is used with misoprostol (PGE1 analog → cervical ripening + uterine contractions) for medical abortion up to 70 days gestation. Mifepristone also has glucocorticoid receptor activity (used in Cushing syndrome at higher doses).

Mifepristone is a competitive progesterone receptor antagonist (also anti-glucocorticoid; weak anti-androgen at high doses). By blocking PR in the myometrium and decidua: (1) uterine quiescence is reversed (progesterone normally suppresses oxytocin receptor expression and gap junction formation), (2) decidual regression occurs, (3) cervical softening/dilation. Misoprostol (PGE1) then amplifies contractions and cervical ripening for complete expulsion. At 9 weeks, the placenta is the progesterone source (not corpus luteum) — so corpus luteum hCG blockade is irrelevant; the PR antagonism at the uterine level is the operative mechanism.

In IVF protocols, GnRH antagonist (or agonist) suppresses pituitary LH to prevent premature ovulation. The corpus luteum requires LH (or hCG) stimulation to sustain progesterone output. With pituitary suppressed, LH is absent → corpus luteum undergoes early luteolysis → progesterone falls → endometrial support fails → implantation failure. Exogenous progesterone bridges this gap until the developing embryo produces hCG and eventually the placenta becomes steroidogenically autonomous (luteal-placental shift ~weeks 8–10). Luteal phase support is standard of care for ALL IVF cycles using GnRH analogues.

This is CAIS. The undescended testes in CAIS carry a significant risk of malignancy (gonadoblastoma, dysgerminoma) — estimated 3–22% lifetime risk. Standard management: allow puberty to complete naturally (the testes are the source of aromatizable testosterone → estrogen → breast development and bone density), THEN perform gonadectomy. After removal: lifelong estrogen replacement to maintain bone density and quality of life. Vaginal dilation may be needed (blind vaginal pouch) for sexual function. Genetic counseling for family members (X-linked recessive). Fertility is not possible (no uterus/ovaries).

In CAIS, undescended (intra-abdominal or inguinal) testes carry ~2–3% lifetime risk of germ cell malignancy (gonadoblastoma in childhood, seminoma/dysgerminoma in adulthood). Current guidelines: consider deferring gonadectomy until after puberty — testicular aromatization of testosterone → estradiol drives natural breast development and bone density accrual without needing exogenous estrogen during puberty. After puberty, gonadectomy + lifelong HRT. These patients are never virilized regardless of testosterone levels (AR is completely nonfunctional), so the indication is purely malignancy prevention, not androgen suppression.