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Difficulty: MediumEndocrinology and Metabolism

An oncologist requests an internal medicine consultation for a 62-year-old male patient who is experiencing sudden-onset lethargy, anorexia, and erectile dysfunction. The patient is currently receiving immune checkpoint inhibitor therapy (ipilimumab and nivolumab) for advanced melanoma. Vital signs include a blood pressure of 96/60 mm Hg and a heart rate of 54/min. He appears mildly lethargic but is oriented to person, place, and time. There is no hyperpigmentation of the palmar creases or oral mucosa.

Recent biochemical testing shows the following results:

Serum sodium: 132 mEq/L
Serum potassium: 4.0 mEq/L
TSH: 0.1 µU/mL (Normal: 0.4–4.2 µU/mL)
Free thyroxine (T4): 0.4 ng/dL (Normal: 0.9–1.7 ng/dL)
Serum cortisol (8:00 AM): 2.8 µg/dL (Normal: 5–25 µg/dL)

Which of the following is the most likely pathophysiologic mechanism explaining this patient's clinical and laboratory presentation?

  1. Lymphocytic infiltration of the adenohypophysisAnswer
  2. B
    Cell-mediated damage to thyroid follicular cells
  3. C
    Primary autoimmune failure of the adrenal cortex
  4. D
    Reduced tissue activity of deiodinase enzymes
  5. E
    Factitious consumption of exogenous levothyroxine

Answer

Lymphocytic infiltration of the adenohypophysis
The correct answer involves inflammation of the anterior pituitary (adenohypophysis). This patient is receiving immune checkpoint inhibitors (CTLA-4 and PD-1 inhibitors), which are notorious for causing immune-related adverse events. Ipilimumab in particular is strongly associated with hypophysitis (lymphocytic infiltration of the pituitary). This condition causes varying degrees of panhypopituitarism. The laboratory profile demonstrates central hypothyroidism (low free T4 with an inappropriately low TSH) and central adrenal insufficiency (low morning cortisol). Unlike primary adrenal insufficiency, central adrenal insufficiency does not typically cause hyperkalemia or severe hyperpigmentation because aldosterone production remains intact (it is primarily regulated by the renin-angiotensin-aldosterone system, not ACTH) and ACTH levels are low.

Step-by-Step Solution

1
Analyze the patient's medication history and new symptoms.
Patient is on immune checkpoint inhibitors (ipilimumab/nivolumab) and presents with fatigue, anorexia, lethargy, and erectile dysfunction.
Checkpoint inhibitors frequently cause immune-related adverse events, particularly autoimmune endocrinopathies.
2
Evaluate the thyroid laboratory pattern.
Free T4 is low (0.4 ng/dL), but TSH is inappropriately low (0.1 µU/mL).
This pattern indicates central (secondary) hypothyroidism, as the pituitary is failing to mount a TSH response to low peripheral thyroid hormone levels.
3
Evaluate the adrenal laboratory pattern and physical exam findings.
Morning cortisol is low, but sodium is normal, potassium is normal, and there is no hyperpigmentation.
This indicates central adrenal insufficiency. ACTH is deficient, but aldosterone (regulated by the renin-angiotensin-aldosterone system) remains intact, maintaining normal potassium levels.
4
Synthesize findings to determine the underlying mechanism.
Combined central hypothyroidism and central adrenal insufficiency in a patient on ipilimumab points directly to a pituitary source.
Ipilimumab induces hypophysitis (lymphocytic infiltration of the adenohypophysis), leading to failure of multiple anterior pituitary hormone axes.

Key Concept

Immune checkpoint inhibitor-induced hypophysitis presents with panhypopituitarism, characteristically demonstrating central hypothyroidism (low TSH, low free T4) and central adrenal insufficiency (low ACTH, low cortisol with preserved aldosterone).
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Topics

Endocrinology and Metabolism

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