A 68-year-old man presents to his primary care physician complaining of frequent urination at night. He reports a strong urinary stream without any feeling of incomplete emptying. His medications include aspirin, pravastatin, lithium, and amlodipine. On physical examination, he is afebrile, his blood pressure 138/75 mm Hg, pulse rate 74 beats/min, and respiratory rate 18 breaths/min.
Which of the following laboratory studies are MOST consistent with the patient’s clinical presentation?
Correct Answer: C
The patient’s history and presentation is consistent with lithium-induced NDI. In NDI, the kidney’s ability to concentrate the urine is decreased despite the presence of sufficient ADH, due to insufficient water reabsorption in the collecting duct. Vasopressin is responsible for regulating the water permeability in the collecting duct and water reabsorption based on the tonicity of the medullary interstitium. Lithium causes NDI by entering the principal cells in the collecting tubules and interfering with aquaporin function, thereby decreasing the ability to concentrate urine.
Other causes for NDI include hypercalcemia, hypercalciuria, and obstructive uropathy. In the setting of a positive water deprivation test, the administration of desmopressin can further differentiate primary polydipsia from central and nephrogenic DI. Patients with NDI will not respond to the administration of desmopressin, whereas patients with central DI will have an increased urine osmolality.
In NDI, the urine osmolarity is typically lower than serum osmolarity (option C). Since water intake is maintained by compensatory polydipsia, patients do not typically develop overt hypernatremia.