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Difficulty: MediumEndocrinology and Metabolism

A 64-year-old man presents to the oncology clinic for an urgent evaluation due to a two-week history of profound fatigue, generalized weakness, and a persistent dull headache. He has metastatic melanoma and has been receiving immunotherapy with ipilimumab and nivolumab for the past three months. His temperature is 37.0∘C37.0^\circ\text{C} (98.6∘F98.6^\circ\text{F}), blood pressure is 100/62 mmHg100/62\text{ mmHg}, and pulse is 92/min92/\text{min}. Physical examination shows normal skin turgor and full visual fields to confrontation. There is no thyromegaly. Laboratory studies show:

* Serum sodium: 132 mEq/L132\text{ mEq/L} (Normal: 136−145 mEq/L136 - 145\text{ mEq/L})
* Serum potassium: 4.2 mEq/L4.2\text{ mEq/L} (Normal: 3.5−5.0 mEq/L3.5 - 5.0\text{ mEq/L})
* Thyroid-stimulating hormone (TSH): 0.2 mU/L0.2\text{ mU/L} (Normal: 0.5−5.0 mU/L0.5 - 5.0\text{ mU/L})
* Free thyroxine (T4): 0.4 ng/dL0.4\text{ ng/dL} (Normal: 0.9−1.7 ng/dL0.9 - 1.7\text{ ng/dL})

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

  1. Autoimmune lymphocytic infiltration of the pituitary glandAnswer
  2. B
    Autoimmune destruction of the thyroid gland follicles
  3. C
    Production of activating autoantibodies against the thyrotropin receptor
  4. D
    Suppressed pituitary hormone production secondary to acute systemic illness
  5. E
    Reduced peripheral tissue sensitivity to circulating thyroid hormones

Answer

Autoimmune lymphocytic infiltration of the pituitary gland
This patient's presentation is highly characteristic of immune checkpoint inhibitor-induced hypophysitis. Ipilimumab (an anti-CTLA-4 monoclonal antibody) and nivolumab (an anti-PD-1 antibody) enhance T-cell activity against tumor cells but frequently cause immune-related adverse events. Hypophysitis (inflammation of the pituitary gland due to autoimmune lymphocytic infiltration) is a classic complication, particularly of ipilimumab. It typically presents with a headache (due to glandular swelling stretching the sella turcica) and signs of anterior pituitary hormone deficiency. The patient's laboratory tests reveal central hypothyroidism (low free T4 with an inappropriately low TSH). Furthermore, his fatigue, hypotension, and hyponatremia strongly suggest concurrent central adrenal insufficiency (decreased ACTH).

Step-by-Step Solution

1
Analyze the thyroid function test results.
The patient has both a low free T4 and a low TSH.
This specific combination indicates central (secondary) hypothyroidism, as the pituitary gland is failing to secrete TSH appropriately in response to low circulating thyroid hormone.
2
Correlate the laboratory findings with the patient's medication history and symptoms.
The patient is taking immune checkpoint inhibitors (ipilimumab and nivolumab) and presents with headache, fatigue, and hyponatremia.
These findings are classic for a specific adverse effect of immunotherapy.
3
Synthesize the data to determine the underlying mechanism.
The patient has developed drug-induced hypophysitis, which is characterized by autoimmune lymphocytic infiltration of the anterior pituitary gland.
Hypophysitis leads to the destruction of pituitary hormone-producing cells, resulting in panhypopituitarism (including central hypothyroidism and central adrenal insufficiency).

Key Concept

Immune checkpoint inhibitors, particularly anti-CTLA-4 agents like ipilimumab, can cause autoimmune hypophysitis. This presents with headache, fatigue, and laboratory evidence of multiple anterior pituitary hormone deficiencies (e.g., central hypothyroidism with low TSH and low T4).
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Topics

Endocrinology and Metabolism

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