Differential Diagnosis for Elevated PTH with Normal Calcium and Phosphorus

Single Most Likely Diagnosis

• Primary Hyperparathyroidism with early or mild disease: In early or mild primary hyperparathyroidism, PTH levels can be elevated while calcium levels remain within the normal range due to the body's compensatory mechanisms. Phosphorus levels can also be normal, especially if the disease is not advanced.

Other Likely Diagnoses

• Vitamin D deficiency: Vitamin D deficiency can lead to secondary hyperparathyroidism, where PTH levels are elevated in response to low vitamin D levels. Calcium and phosphorus levels can be normal, especially if the deficiency is mild or recently developed.
• Chronic kidney disease (CKD) with secondary hyperparathyroidism: In CKD, impaired vitamin D activation and phosphate retention can lead to elevated PTH levels. Early in the disease, calcium and phosphorus levels might still be within the normal range.
• Familial hypocalciuric hypercalcemia (FHH): Although calcium levels are typically elevated in FHH, some family members may have normal calcium levels but elevated PTH. Phosphorus levels are usually normal.

Do Not Miss Diagnoses

• Lithium-induced hyperparathyroidism: Lithium therapy can cause hyperparathyroidism, leading to elevated PTH levels. It's crucial to identify this cause, as it may require adjustment of the patient's medication regimen.
• Parathyroid cancer: Although rare, parathyroid cancer can present with elevated PTH and normal calcium and phosphorus levels, especially if the tumor is not producing a significant amount of PTH-related protein (PTHrP) or if the disease is caught early.

Rare Diagnoses

• Multiple endocrine neoplasia (MEN) syndromes: These syndromes can include primary hyperparathyroidism as one of their components. While the calcium level might be normal in early disease, the presence of other endocrine abnormalities can help in diagnosing these conditions.
• Teriparatide (recombinant PTH) therapy: Patients on teriparatide for osteoporosis will have elevated PTH levels. However, this is an iatrogenic cause and should be easily identifiable based on the patient's medication history.
• Resistance to PTH (pseudohypoparathyroidism): In some forms of pseudohypoparathyroidism, PTH levels can be elevated due to resistance at the tissue level, while calcium and phosphorus levels might remain normal due to the body's compensatory mechanisms.