Which condition is typically associated with hyponatremia due to enhanced ADH activity?

• A. SIADH
• B. DI
• C. Addison's disease
• D. Hyperthyroidism

Answer: (A)

Enhanced ADH activity leads to retention of water, which dilutes the blood and lowers serum sodium. The condition that fits this pattern is SIADH, where ADH is secreted inappropriately even when plasma osmolality is low. That causes the kidneys to reclaim water, producing euvolemic hyponatremia with a low serum osmolality and a still-concentrated urine (urine osmolality relatively high, often with elevated urine sodium due to ongoing natriuresis). In other words, low sodium here comes from too much water being kept in, not from losing sodium.

In contrast, a diabetes insipidus problem would lead to excessive water loss and typically hypernatremia, not hyponatremia. Addison’s disease causes hyponatremia mainly through cortisol deficiency and volume depletion rather than inappropriate ADH activity. Hyperthyroidism can be associated with hyponatremia, but not through the mechanism of ADH-driven water retention that defines SIADH.