In SIADH, which laboratory finding is typically observed?

• A. High serum osmolality
• B. High serum uric acid
• C. Low serum uric acid
• D. Hyperkalemia

Answer: (C)

In SIADH there is inappropriate ADH-driven water retention, which dilutes the serum and lowers serum osmolality while the patient remains euvolemic. The body’s response includes increased renal excretion of uric acid, so serum uric acid levels fall. This makes low serum uric acid a typical lab finding in SIADH. (Urine studies would show a concentrated urine with high urine sodium, reflecting ongoing natriuresis.) Hyperkalemia or high serum osmolality would not fit this setting.