Pituitary Adenoma: A High-Yield USMLE Review

A pituitary adenoma is a benign tumor of the anterior pituitary that produces disease through two distinct mechanisms: hormonal effects (excess or deficiency) and local mass effect. It is a favorite USMLE topic because anatomy directly predicts presentation and because different subtypes (prolactinoma, GH-secreting, ACTH-secreting, non-functioning) demand very different diagnostic and treatment strategies.

Presentation

• Mass effect from superior growth: headaches from pressure on the sella and dura, and bitemporal (peripheral) visual field defects from optic chiasm compression—signaling a macroadenoma extending above the sella
• Lateral growth into the cavernous sinus produces cranial nerve palsies (CN III, IV, V1, V2, VI), which all course through the sinus wall
• GH-secreting adenoma in an adult (closed growth plates) causes acromegaly: enlarged ring and shoe size, coarse facial features, prominent brow, and enlarged jaw
• Prolactinoma: markedly elevated prolactin; non-functioning adenomas may cause hypopituitarism (e.g., central hypogonadism with low testosterone and low/inappropriately normal LH and FSH)
• ACTH-secreting adenoma produces Cushing's disease from cortisol excess
• Pituitary apoplexy: sudden thunderclap headache, visual loss, and altered mental status, with possible acute panhypopituitarism

Diagnosis

• MRI of the pituitary/sella to define tumor size and extension; note that MRI misses ~40% of small ACTH-secreting microadenomas
• Prolactin level to distinguish subtype: very high prolactin (>200) indicates a prolactinoma, whereas a modest elevation with a large tumor suggests stalk effect from a non-functioning adenoma
• Pituitary hormone axis testing—use dynamic testing when baseline levels are equivocal (stimulation tests for deficiency, suppression tests for excess); in central hypothyroidism check Free T4 directly because TSH may be inappropriately normal
• For suspected Cushing's disease: confirm hypercortisolism first (24-hour urine free cortisol, late-night salivary cortisol, 1 mg overnight dexamethasone suppression), then measure plasma ACTH; high-dose (8 mg) dexamethasone suppression—pituitary adenomas suppress (>50% reduction), ectopic ACTH does not
• Inferior petrosal sinus sampling (IPSS) is the gold standard to localize ACTH source: a central-to-peripheral ratio >2 (or >3 after CRH) confirms a pituitary source

Management

• Prolactinomas are treated medically first with dopamine agonists (cabergoline or bromocriptine), which lower prolactin and shrink the tumor; cabergoline is generally preferred, but bromocriptine is favored in women seeking pregnancy due to its longer safety record
• Surgery (transsphenoidal) for prolactinomas is reserved for dopamine agonist intolerance/resistance, cystic tumors, or patient preference; observation is acceptable for asymptomatic microprolactinoma in postmenopausal women without mass effect
• Non-prolactin secreting and mass-effect tumors are managed surgically rather than medically first
• In suspected pituitary apoplexy: give high-dose corticosteroids immediately (treat presumptive adrenal crisis before testing), obtain urgent MRI (or CT), and consider emergent surgery for vision loss
• In hypopituitarism, replace cortisol before thyroid hormone—starting thyroid replacement without adequate cortisol can precipitate a fatal adrenal crisis

High-yield

• Superior growth → optic chiasm → bitemporal hemianopia; lateral growth → cavernous sinus → CN III/IV/V1/V2/VI palsy
• Prolactin >200 = prolactinoma; modest elevation with a large tumor = stalk effect from a non-functioning adenoma
• Prolactinomas are the exception—treated medically (dopamine agonists) rather than surgically first
• Acromegaly = GH-secreting adenoma in an adult with closed growth plates (enlarged ring/shoe size, coarse features, prominent brow, enlarged jaw)
• Cushing's disease (pituitary adenoma) retains feedback and suppresses on high-dose dexamethasone; ectopic ACTH does not
• IPSS is the gold standard for confirming a pituitary ACTH source, especially since MRI misses many microadenomas
• Check calcium in any patient with a pituitary adenoma—if elevated, consider MEN 1 and look for pancreatic tumors
• Pituitary apoplexy = thunderclap headache + visual loss + altered mental status; give stress-dose steroids first

Pitfalls

• Mislabeling a modestly elevated prolactin as a prolactinoma—a level around 50–85 with a macroadenoma reflects stalk effect from a non-functioning adenoma, not true prolactin hypersecretion
• Waiting for lab confirmation of hypocortisolism in suspected apoplexy—give high-dose steroids immediately
• Trusting a 'normal' TSH in pituitary disease; central hypothyroidism shows an inappropriately normal TSH, so check Free T4 directly
• Starting thyroid hormone before cortisol replacement in hypopituitarism, which can be fatal
• Assuming central hypogonadism is testicular—low testosterone with LOW LH/FSH points to the pituitary/hypothalamus, not primary gonadal failure (which raises LH/FSH)
• Relying on MRI alone to exclude a pituitary source of ACTH excess—it misses about 40% of microadenomas