Question:
A patient presents with hyponatremia (128 mEq/L). In which of the following types of lung cancer is this most commonly seen?
A patient presents with hyponatremia (128 mEq/L). In which of the following types of lung cancer is this most commonly seen?
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Lung Cancer Paraneoplastic Associations to memorize:
- Small Cell = SIADH (hyponatremia), Ectopic ACTH (Cushing's), Lambert-Eaton syndrome.
- Squamous Cell = PTHrP (hypercalcemia).
- Adenocarcinoma = Hypertrophic osteoarthropathy (clubbing).
- Small Cell = SIADH (hyponatremia), Ectopic ACTH (Cushing's), Lambert-Eaton syndrome.
- Squamous Cell = PTHrP (hypercalcemia).
- Adenocarcinoma = Hypertrophic osteoarthropathy (clubbing).
Updated On: Sep 3, 2026
- Small cell lung carcinoma
- Large cell lung carcinoma
- Adenocarcinoma of the lung
- Squamous cell carcinoma of the lung
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The Correct Option is A
Solution and Explanation
Concept:
The clinical vignette describes a patient with newly documented hyponatremia (serum sodium of 128 mEq/L, where normal is 135-145 mEq/L).
The clinician must link this specific metabolic derangement to the correct histological subtype of lung cancer known to cause it.
Explanation:
• Hyponatremia occurring in the setting of lung cancer is almost exclusively caused by the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH).
• SIADH is a classic paraneoplastic syndrome where the malignant tumor cells autonomously synthesize and secrete ectopic Antidiuretic Hormone (ADH, or vasopressin).
• The excess circulating ADH acts on the V2 receptors in the renal collecting ducts, causing massive, inappropriate reabsorption of free water into the systemic circulation.
• This free water retention dilutes the serum sodium, leading to a euvolemic, dilutional hyponatremia.
• Among all the histological subtypes of lung cancer, Small Cell Lung Carcinoma (SCLC) is by far the most strongly associated with paraneoplastic endocrine syndromes.
• SCLC is a rapidly growing neuroendocrine tumor, and its cells are highly proficient at producing ectopic hormones.
• Roughly 10% to 15% of all SCLC patients will develop SIADH during their disease course.
• Another classic paraneoplastic syndrome associated with SCLC is ectopic ACTH production (leading to Cushing's syndrome) and Lambert-Eaton myasthenic syndrome.
• Squamous cell carcinoma is classically associated with hypercalcemia (due to ectopic PTHrP secretion), not hyponatremia.
• Adenocarcinoma is sometimes associated with hypertrophic osteoarthropathy or hypercoagulability (Trousseau syndrome).
Final Answer:
Ectopic ADH secretion causing hyponatremia is a classic paraneoplastic syndrome uniquely associated with Small cell lung carcinoma.
The clinical vignette describes a patient with newly documented hyponatremia (serum sodium of 128 mEq/L, where normal is 135-145 mEq/L).
The clinician must link this specific metabolic derangement to the correct histological subtype of lung cancer known to cause it.
Explanation:
• Hyponatremia occurring in the setting of lung cancer is almost exclusively caused by the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH).
• SIADH is a classic paraneoplastic syndrome where the malignant tumor cells autonomously synthesize and secrete ectopic Antidiuretic Hormone (ADH, or vasopressin).
• The excess circulating ADH acts on the V2 receptors in the renal collecting ducts, causing massive, inappropriate reabsorption of free water into the systemic circulation.
• This free water retention dilutes the serum sodium, leading to a euvolemic, dilutional hyponatremia.
• Among all the histological subtypes of lung cancer, Small Cell Lung Carcinoma (SCLC) is by far the most strongly associated with paraneoplastic endocrine syndromes.
• SCLC is a rapidly growing neuroendocrine tumor, and its cells are highly proficient at producing ectopic hormones.
• Roughly 10% to 15% of all SCLC patients will develop SIADH during their disease course.
• Another classic paraneoplastic syndrome associated with SCLC is ectopic ACTH production (leading to Cushing's syndrome) and Lambert-Eaton myasthenic syndrome.
• Squamous cell carcinoma is classically associated with hypercalcemia (due to ectopic PTHrP secretion), not hyponatremia.
• Adenocarcinoma is sometimes associated with hypertrophic osteoarthropathy or hypercoagulability (Trousseau syndrome).
Final Answer:
Ectopic ADH secretion causing hyponatremia is a classic paraneoplastic syndrome uniquely associated with Small cell lung carcinoma.
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