Differential Diagnosis

The patient's laboratory results show elevated calcium levels (11.1 mg/dL) and ionized calcium levels (5.8 mg/dL), with a parathyroid hormone (PTH) level within the normal range (75 pg/mL). Phosphate levels are also elevated (5.5 mg/dL). Based on these results, the following differential diagnoses can be considered:

• Single Most Likely Diagnosis
  • Hypercalcemia of Malignancy: This condition is characterized by elevated calcium levels due to the presence of a malignancy, often with a normal or low PTH level. The normal PTH level in this case, despite high calcium, suggests that the hypercalcemia is not due to primary hyperparathyroidism.
• Other Likely Diagnoses
  • Vitamin D Intoxication: Elevated vitamin D levels can lead to increased absorption of calcium from the gut, resulting in hypercalcemia. Although the vitamin D level is not provided, this diagnosis should be considered, especially if the patient has a history of vitamin D supplementation.
  • Familial Hypocalciuric Hypercalcemia (FHH): This is a rare genetic disorder characterized by hypercalcemia with a normal or elevated PTH level. However, it is less likely given the absence of a family history.
• Do Not Miss Diagnoses
  • Primary Hyperparathyroidism: Although the PTH level is within the normal range, primary hyperparathyroidism can occasionally present with a normal PTH level, especially if the patient has a parathyroid adenoma or hyperplasia. This diagnosis should not be missed, as it requires surgical intervention.
  • Milk-Alkali Syndrome: This condition is characterized by hypercalcemia, alkalosis, and renal impairment, often due to excessive intake of calcium and alkali. Although less likely, it is a potentially life-threatening condition that should not be missed.
• Rare Diagnoses
  • Tertiary Hyperparathyroidism: This condition occurs in patients with chronic kidney disease who develop hyperparathyroidism due to secondary hyperparathyroidism, which eventually becomes autonomous. Although rare, it should be considered in patients with a history of kidney disease.
  • Parathyroid Hormone-Related Protein (PTHrP)-Mediated Hypercalcemia: This condition is often associated with malignancies, such as squamous cell lung cancer or breast cancer, and can cause hypercalcemia with a normal or low PTH level. Although rare, it is an important diagnosis to consider in patients with a history of malignancy.