Differential Diagnosis
The patient presents with progressive abdominal pain, constipation, nausea, chronic cough, significant weight loss, and fatigue. Laboratory studies reveal hypercalcemia, low phosphorus, and low parathyroid hormone (PTH) levels. Based on these findings, the differential diagnosis can be categorized as follows:
- Single most likely diagnosis
- Squamous cell lung cancer: The patient's 30-year history of tobacco use, chronic cough, and decreased breath sounds in the right middle lobe are highly suggestive of lung cancer. Hypercalcemia in the setting of low PTH levels is consistent with paraneoplastic syndrome, specifically humoral hypercalcemia of malignancy, which is often associated with squamous cell lung cancer.
- Other Likely diagnoses
- Multiple myeloma: Although the patient's presentation and laboratory findings are not as strongly suggestive of multiple myeloma, it remains a possible diagnosis given the hypercalcemia and weight loss. However, the low PTH level and absence of other typical findings such as anemia, renal failure, or lytic bone lesions make it less likely.
- Primary hyperparathyroidism: This diagnosis is unlikely due to the low PTH level, which is not consistent with primary hyperparathyroidism.
- Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
- Pneumonia or lung abscess: Although the patient's lung examination and symptoms suggest a possible malignancy, an infectious process such as pneumonia or lung abscess must be considered and ruled out, especially given the patient's chronic cough and decreased breath sounds.
- Mediastinal malignancy: Other mediastinal malignancies, such as lymphoma or thymoma, could also present with similar symptoms and laboratory findings.
- Rare diagnoses
- Vitamin D intoxication: Although rare, vitamin D intoxication can cause hypercalcemia and low PTH levels. However, this diagnosis would require additional history and laboratory findings to support it.
- Familial hypocalciuric hypercalcemia (FHH): This rare genetic disorder can cause hypercalcemia and low PTH levels, but it is unlikely given the patient's age and lack of family history.