Question of the Week
For July 20, 2021
A 45-year-old woman presents in the emergency department for evaluation of weakness, fatigue, and inability to concentrate. She has no significant medical, surgical, social, or family history and takes no medications. She notes that she started a weight-loss diet one week ago that consists entirely of pineapple, toast, and increased hydration with water and diet soda. She has been drinking approximately 4 liters of water or diet soda per day since she started the diet.
She has a blood pressure of 117/82 mm Hg, a heart rate of 95 beats per minute, a respiratory rate of 18 breaths per minute, and a temperature of 37.1°C. Her weight is 84 kg (BMI, 29.1). Physical examination reveals normal skin turgor; no cyanosis, jaundice, or pallor; normal heart, lung, and abdominal findings; normal jugular venous pressure; and no edema. Neurologic examination is within normal limits.
Laboratory findings are as follows:
| | Patient value | Reference range |
Sodium (mEq/liter) | 123 | 136–145 |
Potassium (mEq/liter) | 3.8 | 3.5–5.0 |
Chloride (mEq/liter) | 92 | 98–106 |
Bicarbonate (mEq/liter) | 22 | 21–30 |
Blood urea nitrogen (mg/dL) | 7 | 10–20 |
Creatinine (mg/dL) | 0.6 | 0.6–1.1 |
Serum fasting glucose (mg/dL) | 125 | 70–100 |
Serum osmolality (mOsm/kg) | 254 | 285–295 |
Hematocrit (%) | 33 | 36–46 |
Leukocyte count (per mm3) | 7500 | 4500–11,000 |
Platelet count (per mm3) | 250,000 | 150,000–350,000 |
Lactate (mmol/liter) | 0.6 | 0.6–1.7 |
Thyroid-stimulating hormone (µU/mL) | 3.2 | 0.5–4.7 |
Morning cortisol (µg/dL) | 14.7 | 5–25 |
Urine sodium (mEq/liter) | 15 | Varies with intake |
Urine osmolality (mOsm/kg) | 68 | Varies with intake |
A urine pregnancy test is negative. Urine output is 900 mL over 4 hours.
What is the most likely cause of this patient’s hyponatremia?