Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 25-year-old woman is evaluated in the emergency department for chest pain after a belted motor vehicle accident. She is pregnant at approximately 23 weeks’ gestation. She reports no additional symptoms and is otherwise well. Her only medication is a prenatal vitamin.

On physical examination, the patient is afebrile, blood pressure is 102/62 mm Hg, and pulse rate is 80/min. Pain and bruising over the left chest wall are noted. Abdominal examination findings are consistent with changes of pregnancy.

Laboratory studies are significant for a serum sodium level of 132 mEq/L (132 mmol/L).

Which of the following is the most likely cause of this patient’s low serum sodium level?

A. Excessive water intake
B. Hypotension-induced antidiuretic hormone release
C. Normal physiologic change in pregnancy
D. Syndrome of inappropriate antidiuretic hormone secretion

MKSAP Answer and Critique

The correct answer is C. Normal physiologic change in pregnancy.

Normal physiologic change in pregnancy is the most likely cause of this patient’s low serum sodium level. Mild hyponatremia is common in normal pregnancy due to plasma volume increases with water retention (mediated by an increase in antidiuretic hormone levels) greater than sodium retention. An associated drop in serum osmolality of 8 to 10 mOsm/kg H₂O and serum sodium concentration of 4 to 5 mEq/L (4-5 mmol/L) may occur. As the serum osmolality and sodium concentration decrease, a new set point is maintained, and thirst occurs in response to osmolality (reset osmostat). No treatment is necessary. Other conditions associated with reset osmostat include quadriplegia, tuberculosis, advanced age, psychiatric disorders, and chronic malnutrition.

Primary polydipsia should always be considered in the differential diagnosis of patients with mental illness and hyponatremia, particularly those with schizophrenia who are taking psychotropic drugs. Primary polydipsia presents with hyponatremia, decreased serum osmolality, and decreased urine osmolality, reflecting suppressed antidiuretic hormone (ADH) levels in response to water overload. Primary polydipsia is a rare cause of hyponatremia, and the volume of water intake would need to be very large to induce hyponatremia. This patient is not at risk for primary polydipsia.

Hypovolemia causes stimulation of the sympathetic nervous system, activation of the renin-angiotensin-aldosterone axis, and release of ADH. These adaptive responses allow volume maintenance at the expense of a low serum sodium with excessive water intake. Blood pressure in pregnant women begins to lower in the first trimester and reaches a nadir in the second. Furthermore, she is asymptomatic, and ADH release is therefore not likely to be induced by this level of blood pressure.

The syndrome of inappropriate antidiuretic hormone (SIADH) secretion may be associated with stress and pain; however, hyponatremia does not develop acutely. Although SIADH could have preceded the patient’s car accident, she has no risk factors for SIADH (central nervous system disorders, pulmonary disorders, infection, drugs, postoperative status, tumors), and normal pregnancy is a more likely cause of her low serum sodium level.

Key Point

• Mild hyponatremia is common in normal pregnancy due to plasma volume increases with water retention greater than sodium retention; no treatment is necessary.

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