A client with severe head trauma sustained in a car accident is admitted to the intensive care unit. Thirty-six hours later, the client’s urine output suddenly rises above 200 ml/hour, leading the nurse to suspect diabetes insipidus. Which laboratory findings support the nurse’s suspicion of arginine vasopressin deficiency?

Reasoning:

Choice A reason: Diabetes insipidus is likely due to the client’s symptoms of polyuria, thirst, and weight loss following a head injury. Trauma can disrupt the posterior pituitary, reducing ADH secretion, leading to excessive dilute urine output, dehydration, and subsequent thirst and weight loss from fluid depletion, consistent with central DI.

Choice B reason: A pituitary tumor may cause diabetes insipidus but is not the condition itself. Tumors can disrupt ADH production, but the symptoms described—polyuria, thirst, and weight loss—point directly to diabetes insipidus as the primary condition, with a tumor being a potential underlying cause requiring further investigation.

Choice C reason: Hypothyroidism, caused by thyroid hormone deficiency, leads to symptoms like fatigue and weight gain, not polyuria or weight loss. It is unrelated to head injury or ADH dysfunction, making it an unlikely diagnosis for the client’s symptoms of excessive urine output and dehydration.

Choice D reason: SIADH causes water retention, leading to concentrated urine, hyponatremia, and potential weight gain, opposite to the client’s symptoms of dilute urine, weight loss, and thirst. Head injury may cause SIADH, but the clinical presentation aligns with diabetes insipidus, not water retention.

Reasoning:

Choice A reason: A blood pressure reading of 120/85 mm Hg is normal but not specific to SIADH. While fluid overload in SIADH may elevate blood pressure, this reading is not diagnostic. Hypertension is possible but not a consistent finding, as fluid retention primarily causes hyponatremia and other symptoms.

Choice B reason: Pitting edema in the lower extremities is uncommon in SIADH, as fluid retention is primarily intravascular, leading to dilutional hyponatremia rather than extravascular edema. Edema is more typical in conditions like heart failure or nephrotic syndrome, not the water retention mechanism of SIADH.

Choice C reason: Normal skin turgor is not typical in SIADH, as water retention can cause slight fluid overload, potentially leading to subtle tissue swelling. While not as pronounced as edema, skin turgor may be slightly increased due to excess fluid, making “normal” less accurate than moist mucous membranes.

Choice D reason: Moist mucous membranes are expected in SIADH due to excessive water retention from ADH overactivity. This leads to fluid overload, keeping mucosal tissues hydrated and moist, unlike the dehydration seen in diabetes insipidus, which causes dry mucous membranes due to water loss.