A 32-year-old primigravida at 16 weeks gestation comes to the office due to urinary frequency and increased thirst. Although she limits fluids before bed, she still urinates 6-8 times a night. The patient previously had intermittent urinary frequency, but symptoms have worsened since she became pregnant. Family history is significant for type 2 diabetes mellitus in her mother and father. Vital signs are within normal limits. Examination shows no abnormalities. Laboratory results are as follows: A 24-hour urine collection shows a urine output of 5.5 L after an intake of 2 L. What is the most likely cause of this patient's urinary frequency?

Question

A 32-year-old primigravida at 16 weeks gestation comes to the office due to urinary frequency and increased thirst.  Although she limits fluids before bed, she still urinates 6-8 times a night.  The patient previously had intermittent urinary frequency, but symptoms have worsened since she became pregnant.  Family history is significant for type 2 diabetes mellitus in her mother and father.  Vital signs are within normal limits.  Examination shows no abnormalities.  Laboratory results are as follows:  A 24-hour urine collection shows a urine output of 5.5 L after an intake of 2 L.  What is the most likely cause of this patient's urinary frequency?

Quizplus · Accepted Answer

11ec25c8_d998_0ca3_bc81_4d80f14ad474_MD0003_00 This pregnant patient with polydipsia and nocturia has polyuria (eg, 24-hr urinary output >3 L), and her urinalysis reveals low specific gravity (<1.006), indicating dilute urine (water diuresis). Polyuria and dilute urine in the setting of normal serum sodium and fluid intake (2 L) suggest diabetes insipidus (DI). DI is characterized by an inability to concentrate urine and is due to an insufficiency of, or renal resistance to, antidiuretic hormone (ADH, or vasopressin), which normally increases renal water reabsorption in the collecting ducts. DI can remain undiagnosed because the body often compensates for water losses with a demand for increased fluid intake (eg, increased thirst). However, during pregnancy, placentally produced enzymes (eg, vasopressinase) increase ADH breakdown and can worsen symptoms, thereby unmasking undiagnosed DI. Some women develop transient DI of pregnancy due to an exaggerated response to vasopressinase. This phenomenon may resolve with delivery but recur in subsequent pregnancies. In this patient, a more complete workup (eg, challenge with desmopressin, an ADH analogue) is required to confirm the precise etiology of DI (ie, central DI due to insufficient ADH levels vs nephrogenic DI due to renal resistance to ADH). (Choices B and D) Polyuria occurs in gestational diabetes mellitus and urea-related osmotic diuresis. However, polyuria in both settings results from osmotic diuresis (eg, excretion of excess glucose due to hyperglycemia or of urea following kidney injury resolution). Therefore, a high specific gravity, indicating concentrated urine, would be expected. This patient's low urine specific gravity, normoglycemia, and absence of kidney injury make osmotic diuresis unlikely. (Choice C) Increased urinary frequency is common in pregnancy due to physiologic changes and expanded blood volume. However, normal pregnancy changes are not associated with overt polyuria. This patient's excessive urinary output (despite normal fluid intake) and low urine specific gravity suggest an underlying disorder. (Choice E) Urinary tract infection can manifest with urinary frequency and urgency but does not cause pathologically increased urinary output (eg, polyuria). Furthermore, this patient's urinalysis result (absent bacteriuria, normal nitrite, normal leukocyte esterase) make infection unlikely. Educational objective: Diabetes insipidus (DI) is characterized by polyuria (eg, 24-hr urinary output >3 L) and excretion of an inappropriately dilute urine with low specific gravity despite normal serum sodium concentration and fluid intake. During pregnancy, preexisting mild DI symptoms can worsen due to increased turnover of antidiuretic hormone.

A 32-Year-Old Primigravida at 16 Weeks Gestation Comes to the Office