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Difficulty: MediumEndocrinology and Metabolism

A 5656-year-old man comes to the clinic reporting profound fatigue, generalized weakness, and feeling cold all the time for the past 33 weeks. He has a history of metastatic melanoma and has been receiving ipilimumab therapy for the past 44 months. He reports poor appetite but has not lost significant weight. Blood pressure is 98/60 mm Hg98/60\text{ mm Hg} and pulse is 58/min58\text{/min} with no orthostatic changes. Physical examination shows pale skin without hyperpigmentation and delayed relaxation of the deep tendon reflexes. Laboratory studies show the following:

Serum sodium: 138 mEq/L138\text{ mEq/L}
Serum potassium: 4.1 mEq/L4.1\text{ mEq/L}
Serum thyroid-stimulating hormone (TSH): 1.1 mU/L1.1\text{ mU/L} (normal: 0.4–4.0 mU/L0.4\text{--}4.0\text{ mU/L})
Serum free thyroxine (T4): 0.5 ng/dL0.5\text{ ng/dL} (normal: 0.8–1.8 ng/dL0.8\text{--}1.8\text{ ng/dL})

Which of the following is the most likely underlying mechanism for this patient's current presentation?

  1. Lymphocytic infiltration of the anterior pituitary glandAnswer
  2. B
    Autoimmune destruction of thyroid follicular cells
  3. C
    Decreased peripheral conversion of thyroxine to triiodothyronine
  4. D
    Metastatic destruction of the bilateral adrenal glands
  5. E
    Circulating antibodies stimulating the thyrotropin receptor

Answer

The correct answer identifies lymphocytic infiltration of the anterior pituitary gland (hypophysitis) as the mechanism.
The correct answer identifies lymphocytic infiltration of the anterior pituitary gland, which is the underlying mechanism of hypophysitis. This patient is being treated with ipilimumab, a CTLA-4 immune checkpoint inhibitor that carries a high risk of immune-related adverse events, particularly hypophysitis. Inflammation of the pituitary gland leads to decreased secretion of anterior pituitary hormones, including thyroid-stimulating hormone (TSH) and adrenocorticotropic hormone (ACTH). The laboratory pattern of a low free T4 combined with a non-elevated (inappropriately normal) TSH is the classic hallmark of central hypothyroidism. Furthermore, the patient's hypotension and profound fatigue point to concurrent central adrenal insufficiency. Crucially, because aldosterone production is regulated by the renin-angiotensin-aldosterone system rather than ACTH, the patient's potassium levels remain normal, and the lack of elevated ACTH means there is no skin hyperpigmentation.

Step-by-Step Solution

1
Analyze the patient's thyroid laboratory results.
The patient has a low free T4 (0.5 ng/dL0.5\text{ ng/dL}) with an inappropriately normal TSH (1.1 mU/L1.1\text{ mU/L}).
This combination indicates a central (secondary) etiology for the hypothyroidism, as a primary thyroid problem would result in an elevated TSH due to the lack of negative feedback.
2
Correlate the central hypothyroidism with the patient's symptoms and medication history.
The patient is taking ipilimumab, an immune checkpoint inhibitor known to cause immune-related adverse events.
Ipilimumab specifically has a high risk of inducing hypophysitis (lymphocytic infiltration of the pituitary gland), leading to anterior pituitary hormone deficiencies.
3
Evaluate the remaining symptoms and lab values to confirm the diagnosis.
The patient has hypotension and fatigue (suggesting adrenal insufficiency), but normal sodium and potassium levels without hyperpigmentation.
Central adrenal insufficiency (due to low ACTH from hypophysitis) preserves aldosterone secretion because aldosterone is primarily regulated by the renin-angiotensin-aldosterone system, thereby keeping potassium levels normal and preventing severe hyponatremia.

Key Concept

Immune checkpoint inhibitor-induced hypophysitis typically presents with central hypothyroidism (low free T4, normal/low TSH) and central adrenal insufficiency (low cortisol, normal electrolytes).
Estimated Time:1m 15s
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Topics

Endocrinology and Metabolism

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