A 16-year-old girl is brought to the clinic by her mother because of frequent urination for the past 2 months. She denies dysuria, urinary urgency, and hematuria. She denies any significant changes in appetite or weight and has not had any visual changes. Past medical history is negative, and she does not take any medications. She does not drink alcohol, smoke cigarettes, or use illicit drugs. Family history is unremarkable. Vital signs are temperature 36.9°C (98.4°F), blood pressure 112/70 mmHg, pulse 91 beats/min, and respirations 15/min. Physical examination is unremarkable. Laboratory studies reveal the following:
Which of the following best explains the underlying etiopathogenesis of this patient’s condition?
C) Excessive fluid intake
This patient’s laboratory findings—mild hyponatremia, low serum osmolality, and inappropriately low urine osmolality—are consistent with primary (psychogenic) polydipsia. In this condition, the excessive intake of water leads to dilution of serum sodium and a reduction in plasma osmolality, prompting the kidneys to excrete very dilute urine in an attempt to restore serum osmolality to normal limits. The kidneys themselves are not diseased, and the body’s production of antidiuretic hormone (ADH) is intact. Restricting fluid intake (water deprivation test) typically normalizes serum and urine osmolality and is used for both diagnosis as well as treatment. Patients with primary polydipsia may have an underlying psychiatric illness or a lesion in the hypothalamic thirst center that is driving excessive fluid intake.
Answer choice A: Autoimmune destruction of pancreatic endocrine cells, is incorrect. This describes type 1 diabetes mellitus, in which autoimmune processes target insulin-producing β-cells. Other than excessive urination, this patient does not have any characteristic symptoms of type 1 diabetes mellitus, such as polyphagia, weight loss, or visual changes. Polyuria in diabetes mellitus is caused by osmotic diuresis from hyperglycemia rather than low serum osmolality.
Answer choice B: Ectopic secretion of antidiuretic hormone, is incorrect. Ectopic secretion of ADH (e.g., by a small cell lung carcinoma) causes syndrome of inappropriate antidiuretic hormone (SIADH). Excessive ADH leads to increased water retention by the kidneys. This leads to a reduction in serum osmolality and serum sodium (dilutional hyponatremia), along with a rise in urine osmolality. Additionally, patients with SIADH do not have polyuria.
Answer choice D: Impaired arginine vasopressin secretion, is incorrect. Arginine vasopressin deficiency (AVP-D, formerly central diabetes insipidus) is caused by inadequate ADH (also known as arginine vasopressin; AVP) secretion from the posterior pituitary. Patients typically have a persistent inability to concentrate urine, leading to production of large amounts of dilute urine with a consequent increase in serum osmolality.
Answer choice E. Renal arginine vasopressin resistance, is incorrect. Arginine vasopressin resistance (AVP-R, formerly nephrogenic diabetes insipidus) involves resistance of the kidneys to ADH. As in AVP-D, these patients cannot appropriately concentrate their urine and produce large amounts of dilute urine with a consequent rise in serum osmolality.
Key Learning Point
Primary (psychogenic) polydipsia results from excessive fluid intake leading to low serum osmolality and mild hyponatremia along with the production of excessive amounts of dilute urine. The kidneys are functionally normal and will concentrate urine appropriately once fluid intake is reduced.
