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 provided (5.5 mg/dL, but without a clear reference range in the question, its interpretation is limited). Based on these findings, the differential diagnosis can be organized into the following categories:

• Single Most Likely Diagnosis
  • Hypercalcemia of Malignancy: This condition is characterized by elevated calcium levels, often with a suppressed or low-normal PTH level, which fits the patient's profile. Malignancies can produce parathyroid hormone-related protein (PTHrP), leading to hypercalcemia without an elevated PTH level.
• Other Likely Diagnoses
  • Familial Hypocalciuric Hypercalcemia (FHH): Although PTH is within the normal range, FHH could be considered, especially if the patient has a family history. However, the calcium levels are quite high, which might be less typical for FHH.
  • Vitamin D Intoxication: Elevated levels of vitamin D can lead to increased absorption of calcium from the gut, resulting in hypercalcemia. The normal PTH level does not rule out this diagnosis, as vitamin D intoxication can suppress PTH secretion.
• Do Not Miss Diagnoses
  • Primary Hyperparathyroidism with PTH Level at the Upper Limit of Normal: Although the PTH level is within the normal range, it is at the upper limit. Given the significantly elevated calcium levels, primary hyperparathyroidism cannot be entirely ruled out without further evaluation, such as a repeat PTH measurement or imaging studies.
  • Milk-Alkali Syndrome: This condition, caused by excessive intake of calcium and alkali, can lead to hypercalcemia. While the PTH level might be expected to be low, it could be in the lower end of the normal range due to the body's attempt to compensate for the hypercalcemia.
• Rare Diagnoses
  • Thyroid Hormone Excess: Hyperthyroidism can cause hypercalcemia due to increased bone turnover. However, this would typically be accompanied by other symptoms of hyperthyroidism.
  • Adrenal Insufficiency: Rarely, adrenal insufficiency can present with hypercalcemia. This diagnosis would be considered if there were other signs or symptoms suggestive of adrenal insufficiency.

Next Steps

Given the differential diagnoses, the next steps could include:

• Further history and physical examination to look for signs of malignancy, familial disorders, or symptoms suggestive of hyperthyroidism or adrenal insufficiency.
• Laboratory tests such as vitamin D levels, phosphate (with a clear reference range), and markers of bone turnover.
• Imaging studies (e.g., chest X-ray, abdominal CT) to evaluate for malignancy or other causes of hypercalcemia.
• Consideration of a 24-hour urinary calcium excretion test to help differentiate between FHH and primary hyperparathyroidism.
• Possibly, a parathyroid scan or biopsy if primary hyperparathyroidism is strongly suspected after initial evaluations.