Pituitary Disorders for Step 1: Adenomas, SIADH, DI

Creator: Progress Note
Published: 2026-10-08
Keywords: pituitary disorders USMLE Step 1, pituitary disorders usmle, prolactinoma cabergoline, acromegaly igf-1, bitemporal hemianopia pituitary, siadh vs diabetes insipidus, water deprivation test, sheehan syndrome, craniopharyngioma, hypopituitarism

bolt The 60-second version

check_circleDopamine from the hypothalamus tonically inhibits prolactin. Dopamine antagonists (antipsychotics, metoclopramide) and stalk compression raise prolactin.
check_circleProlactinoma is the most common pituitary adenoma: galactorrhea, amenorrhea, infertility, low libido. Dopamine agonists (cabergoline, bromocriptine) are first-line treatment.
check_circleAcromegaly: excess GH raises IGF-1 → enlarged hands, feet, and jaw, organomegaly, diabetes, cardiomyopathy. GH fails to suppress after oral glucose.
check_circleA large adenoma compresses the optic chiasm → bitemporal hemianopia.
check_circleSIADH: euvolemic hyponatremia with inappropriately concentrated urine; central DI and nephrogenic DI cause dilute polyuria and high-normal or high sodium.
check_circleSheehan syndrome: pituitary infarction after postpartum hemorrhage → failure to lactate and hypopituitarism.

target What you'll be able to do · aligned to USMLE Step 1 · Endocrine System

ENDODescribe the hypothalamic-pituitary axes and their feedback loops.
ENDORecognize pituitary adenomas and their hormonal and mass effects.
ENDODistinguish SIADH, central DI, nephrogenic DI, and primary polydipsia.
ENDOIdentify causes and treatment priorities in hypopituitarism.

How the pituitary is tested on Step 1

Trace each axis from hypothalamus to target gland and back.

Hypothalamus	Pituitary hormone	Target and feedback
CRH	ACTH	Adrenal cortisol; cortisol inhibits CRH and ACTH
TRH	TSH (and prolactin)	Thyroid T4 and T3; T3 inhibits TRH and TSH
GnRH (pulsatile)	LH, FSH	Gonads; sex steroids and inhibin give feedback
GHRH (+) and somatostatin (−)	GH	Liver IGF-1; IGF-1 inhibits GH
Dopamine (−)	Prolactin	Breast; prolactin inhibits GnRH
Supraoptic and paraventricular nuclei	ADH, oxytocin (posterior pituitary)	Kidney collecting duct (V2); uterus and breast

Continuous GnRH (e.g., leuprolide) suppresses LH and FSH; pulsatile GnRH stimulates them.

Pituitary adenomas and mass lesions

Functioning adenomas cause hormone excess; large ones cause mass effect.

Pituitary masses. Pituitary apoplexy (hemorrhage into an adenoma) causes sudden headache, vision loss, and adrenal crisis.

Hypopituitarism

Loss of several anterior pituitary hormones, from a mass, infarction, surgery, or radiation.

Hypopituitarism: GH and gonadotropins are usually lost first, then TSH and ACTH. Replace cortisol before thyroid hormone to avoid adrenal crisis.
Sheehan syndrome: the pituitary enlarges in pregnancy, so hypotension from postpartum hemorrhage can infarct it.
Prolactin is the exception: stalk damage raises prolactin (loss of dopamine inhibition), while infarction of the gland lowers it.

SIADH and diabetes insipidus

Compare urine concentration with serum sodium.

Water balance. Correct chronic hyponatremia slowly to avoid osmotic demyelination syndrome.

Test	Central DI	Nephrogenic DI	Primary polydipsia
Urine osmolality after water deprivation	Stays low	Stays low	Rises
Response to desmopressin	Urine concentrates	Little change	Already concentrated
Treatment	Desmopressin	Thiazides, low-salt diet; amiloride for lithium	Reduce water intake

Drugs that cause SIADH include carbamazepine, cyclophosphamide, and SSRIs.

Worked example

Read the water deprivation test.

functions Worked example — polyuria after head injury

After a car crash with head trauma, a patient makes 6 L of dilute urine per day. Serum sodium is 147 mEq/L. Urine osmolality stays low after water deprivation but rises sharply after desmopressin. What is the diagnosis?

Large volumes of dilute urine with high-normal sodium suggest diabetes insipidus.
Water deprivation does not concentrate the urine, so this is not primary polydipsia.
The kidney responds to desmopressin, so its V2 receptors work.
The problem is too little ADH from the hypothalamus or posterior pituitary after trauma.

Answer. Central diabetes insipidus; treat with desmopressin.

Next in the roadmap

Pituitary & Hypothalamus

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quiz Test yourself

Quick check — 3 questions

1. A woman has amenorrhea, galactorrhea, and a 6-mm pituitary mass. First-line treatment?

2. A smoker has hyponatremia, normal volume status, and concentrated urine. Most likely cause?

3. After a postpartum hemorrhage, a woman cannot breastfeed and later has fatigue and cold intolerance. Diagnosis?

Frequently asked questions

Why do antipsychotics cause galactorrhea?expand_more

They block D2 receptors, removing dopamine's inhibition of prolactin release.

How is acromegaly confirmed?expand_more

Elevated IGF-1, and GH that does not suppress after an oral glucose load, then pituitary MRI.

What visual field defect does a pituitary adenoma cause?expand_more

Bitemporal hemianopia, from compression of the optic chiasm.

How do central and nephrogenic DI differ on testing?expand_more

Both have dilute urine after water deprivation; only central DI concentrates urine after desmopressin.

References & further reading

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