Which of the following is a common cause of hypotonic euvolemic hyponatremia?

• A. SIADH
• B. Adrenal insufficiency
• C. Water intoxication
• D. Hypothyroidism

Answer: (A)

The key idea is that low sodium with low plasma osmolality and a normal (euvolemic) fluid status is most often due to inappropriate ADH secretion. In SIADH, ADH stays high despite low serum osmolality, so the body retains water, diluting the serum sodium. The result is hyponatremia with a euvolemic presentation because the body compensates enough with natriuresis to avoid edema or dehydration. Lab clues include a low serum osmolality, urine that is inappropriately concentrated (high urine osmolality), and high urine sodium, with normal thyroid and adrenal function.

Adrenal insufficiency and hypothyroidism can cause hyponatremia, but they’re less commonly the cause of a straightforward hypotonic euvolemic hyponatremia, and water intoxication is a less typical mechanism in most exam scenarios.