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Differential Diagnosis for Hypercalcemia and Low iPTH with Normal Vitamin D in a 45-year-old Female

Single Most Likely Diagnosis

  • Malignancy-associated hypercalcemia: This is often the most common cause of hypercalcemia in adults, particularly in the context of low iPTH (intact parathyroid hormone) and normal vitamin D levels. Tumors can produce parathyroid hormone-related protein (PTHrP), leading to hypercalcemia.

Other Likely Diagnoses

  • Familial hypocalciuric hypercalcemia (FHH): Although less common, FHH is a consideration, especially if there's a family history. It's characterized by hypercalcemia, low urinary calcium excretion, and often, low iPTH levels.
  • Vitamin D intoxication is less likely given normal vitamin D levels, but Vitamin A intoxication: Can cause hypercalcemia due to increased bone resorption.
  • Thyroid disorders: Hyperthyroidism can lead to hypercalcemia due to increased bone turnover, though iPTH levels might not always be low.

Do Not Miss Diagnoses

  • Primary hyperparathyroidism with assay interference: Although iPTH is reported as low, certain assays can be interfered with by heterophile antibodies or other substances, potentially masking true primary hyperparathyroidism.
  • Multiple myeloma: A type of blood cancer that can cause hypercalcemia through various mechanisms, including the production of osteoclast-activating factors.
  • Pheochromocytoma: Rarely, can cause hypercalcemia, possibly through PTHrP production or other mechanisms.

Rare Diagnoses

  • Hypercalcemia of malignancy due to ectopic PTH production: Very rare, as most malignancy-associated hypercalcemia is due to PTHrP.
  • Lithium-induced hypercalcemia: Lithium therapy can increase PTH secretion, but this would typically be associated with elevated iPTH levels. However, in some cases, the effect on PTH and calcium levels can be complex.
  • Immobilization hypercalcemia: More common in patients with high bone turnover rates (e.g., young patients with spinal cord injuries), but could be considered in any patient with prolonged immobilization.

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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