Objective The incidence of diabetes mellitus in patients with primary hyperparathyroidism and, conversely, primary hyperparathyroidism in diabetic patients are approximately threefold higher than the respective expected prevalence in the general populace. The diagnosis is straightforward when the patient presents hypercalcemia and inappropriately elevated serum parathyroid hormone (PTH) levels. We report a case of parathyroid adenoma in a diabetic patient with persistent hypercalcemia and normal PTH levels. Patient A 50-year-old female patient who was referred to our outpatient clinic presented with persistent hypercalcemia (serum Ca levels between 10.5 and 11 mg/dl) with a normal serum intact PTH level of 46.1 pg/ml. Her blood pressure was 120/80 mmHg, and she was being treated with antihypertensive therapy. Her HbA1c was 7.2%, and her triglycerides were in the normal range. A bone densitometry exam revealed osteopenia of radius -1.39, femoral neck -1.39, and the total hip -1.04. A neck ultrasound revealed a mass of 13 mm next to the inferior and posterior of the right thyroid lobe. A dual phase Tc-99m-sestamibi scan revealed an area of increased uptake in the same region, which is indicative of a parathyroid adenoma. The parathyroid adenoma was removed, which resulted in the achievement of normocalcemia. Conclusion Diabetic patients should be evaluated for hyperparathyroidism as associated hypertension can complicate the course of the disease. These patients should be evaluated for primary hyperparathyroidism when they exhibit persistent hypercalcemia and when clinical suspicion is aroused even if the serum PTH levels are within the normal range.