The patient's symptoms of excessive thirst (polydipsia), frequent urination (polyuria), and unexplained weight loss are classic signs of diabetes mellitus. This condition is characterized by impaired insulin secretion or action, leading to elevated blood glucose levels (hyperglycemia). High glucose levels increase osmotic diuresis, causing dehydration and triggering thirst, while the inability of glucose to enter cells results in weight loss due to cellular starvation. Therefore, the most likely laboratory finding is hyperglycemia.

Reference: Harrison's Principles of Internal Medicine, Jameson et al., 20th Edition.