Which of the following is NOT a common cause of SIADH?

• A. Small cell lung cancer
• B. Intracranial pathology
• C. Lung disease
• D. Hypothyroidism

Answer: (D)

SIADH comes from inappropriately increased ADH activity, which causes the kidneys to retain water and leads to dilutional hyponatremia with low serum osmolality and concentrated urine. The most common triggers are conditions that drive non-osmotic ADH release: malignancies, especially small cell lung cancer; intracranial disorders such as stroke, head injury, infections, or neurosurgery; and various lung diseases like pneumonia or COPD. Hypothyroidism, while it can be associated with hyponatremia, does not typically cause SIADH via excess ADH; when hyponatremia occurs in hypothyroidism, it’s usually due to other mechanisms like reduced renal perfusion and impaired free-water clearance rather than SIADH. Thus hypothyroidism is not a common cause of SIADH.