A 44-year-old woman comes to the office due to excessive thirst and urination. She has to get up several times during the night to urinate. Two months ago, she was diagnosed with type 2 diabetes mellitus, which she manages with diet and exercise. Blood pressure is 134/78 mm Hg and pulse is 88/min. BMI is 32 kg/m². The remainder of the physical examination is normal. Laboratory results are as follows: Which of the following is the most likely cause for this patient's excessive thirst and urination?

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Explanation: 11ec25c8_d3a7_f77d_bc81_a50413af503f_MD0003_00 This patient with high serum osmolality and urine studies showing low osmolality and specific gravity likely has diabetes insipidus (DI). DI can be caused by either deficient antidiuretic hormone (ADH) production (central DI) or renal ADH resistance (nephrogenic DI). Both central and nephrogenic DI are characterized by impaired water reabsorption and concentration of urine, resulting in high serum osmolality and polyuria with dilute urine. In this patient: Polyuria (eg, >3 L urine/24 hours) is suggested by frequent urination and nocturia. Mild hypernatremia is present; this common finding in DI reflects ongoing water losses and maintains a stimulus for compensatory polydipsia (in patients with intact thirst reflex). Low urine osmolality (<300 mOsm/kg H₂O) and low urine specific gravity (<1.006) in the setting of high serum osmolality (>250 mOsm/kg H₂O) reflect inappropriately dilute urine. Polyuria with high serum osmolality can also be seen in diabetes mellitus (DM) complicated by marked hyperglycemia (eg, diabetic ketoacidosis). However, DM causes osmotic diuresis (eg, glucosuria) with a high urinary solute concentration. Therefore, urine specific gravity and urine osmolality (eg, typically >600 mOsm/kg H₂O) are high in DM. This patient's low urine osmolality, low specific gravity, and only mildly elevated glucose levels make DI a much more likely cause of her urinary symptoms (Choice C). ₂ (Choice B) The syndrome of inappropriate ADH production (SIADH) is characterized by excessive ADH production. This causes concentrated urine; polyuria is not seen, and patients typically have hyponatremia. (Choice D) Hypothyroidism can cause hypotonic hyponatremia resulting from decreased free water excretion. It is not associated with the hypernatremia and persistently dilute urine seen in this patient. (Choice E) Impaired thirst perception can cause primary polydipsia (seen in psychiatric conditions or CNS lesions), resulting in excessive water intake and dilute urine. However, patients have low serum osmolality and sodium (hyponatremia). This patient has hyperosmolality and hypernatremia with dilute urine; therefore, her polydipsia reflects an appropriate thirst reflex for replacing ongoing water losses. Educational objective: Urine osmolality and specific gravity can be used to narrow the cause of suspected polyuria. Low urine osmolality and low urine specific gravity (eg, <1.006) suggest dilute urine, as seen in diabetes insipidus. High urine osmolality and high urine specific gravity suggest osmotic diuresis from conditions such as diabetes mellitus. References: Diabetes insipidus. (http://www.ncbi.nlm.nih.gov/pubmed/30819684)

A 44-Year-Old Woman Comes to the Office Due to Excessive