The condition the findings suggest
He has small cell lung cancer and takes sertraline, and his laboratory results show serum sodium 124 mEq/L, serum osmolality 258 mOsm/kg, urine osmolality 520 mOsm/kg, and urine sodium 48 mEq/L, with normal thyroid function. Low serum sodium and osmolality with inappropriately concentrated urine, a urine sodium above 30 mEq/L, normal volume status, and normal thyroid function fit the syndrome of inappropriate antidiuretic hormone (SIADH). Small cell lung cancer and sertraline are both known causes.
The mechanism of SIADH
Antidiuretic hormone (ADH), also called arginine vasopressin, makes the collecting ducts reabsorb water. In SIADH, ADH is released despite low serum osmolality, either from tumor cells (small cell lung cancer can produce it) or because of drugs such as selective serotonin reuptake inhibitors. The kidneys retain free water, which dilutes the serum sodium. The urine is inappropriately concentrated for the low serum osmolality, and urine sodium stays high because the body is not volume-depleted. The client looks euvolemic: blood pressure is normal, mucous membranes are moist, and there is no edema. Headache and confusion reflect brain swelling from the water shift.
| Condition | Serum sodium | Urine | Other clues |
|---|
| SIADH | Low | Concentrated; urine sodium above 30 | Euvolemic |
| Vasopressin deficiency | High | Large volumes, dilute | Thirst, polyuria |
| Low-volume hyponatremia | Low | Urine sodium below about 30 | Dry mucosa, hypotension |
| Primary adrenal insufficiency | Low | Variable | Hypotension, high potassium |
Why the other conditions do not fit
Arginine vasopressin deficiency (central diabetes insipidus) produces large volumes of dilute urine and a high serum sodium. Low-volume hyponatremia would produce dry mucous membranes and a urine sodium below about 30 mEq/L as the kidneys conserve sodium. Primary adrenal insufficiency usually causes hypotension and hyperkalemia; his blood pressure and potassium are normal.
Nursing application
The nurse anticipates fluid restriction, close monitoring of sodium, neurologic status, intake and output, and daily weight, and seizure precautions if sodium falls further. The nurse reports worsening confusion promptly.
Exam takeaway
Key point SIADH: low serum sodium and osmolality, concentrated urine, high urine sodium, and normal volume status. Think small cell lung cancer and SSRIs as causes.