What laboratory measurement most supports the diagnosis of diabetes insipidus after pituitary surgery?

The most relevant laboratory measurement for supporting the diagnosis of diabetes insipidus, particularly in the context of pituitary surgery, is a serum sodium level of 145 meq/L. This elevated sodium level indicates hypernatremia, which is commonly observed in diabetes insipidus due to the inability of the kidneys to concentrate urine. When the body is unable to retain water, excessive fluid loss through urine occurs, resulting in higher serum sodium levels as a consequence of dehydration.

In the context of pituitary surgery, the destruction or dysfunction of vasopressin (antidiuretic hormone, ADH) secretion may lead to diabetes insipidus. Since vasopressin plays a crucial role in regulating water balance and promoting the reabsorption of water in the kidneys, its absence results in dilute urine and increased serum sodium concentrations. Therefore, the finding of elevated serum sodium provides important diagnostic evidence for diabetes insipidus following pituitary surgery.

Other measurements, such as urine osmolarity levels, would provide information about urine concentration but may not directly indicate the presence of diabetes insipidus as clearly as the serum sodium level does, particularly in this post-surgical context.