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?

Cerebral salt wasting
Low-solute diet
Diabetes insipidus
Syndrome of inappropriate antidiuretic hormone
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