Familial Hypocalciuric Hypercalcemia (FHH): This condition is often considered in the differential diagnosis of primary hyperparathyroidism due to its similarity in presentation, including hypercalcemia and elevated parathyroid hormone (PTH) levels. However, FHH typically has a family history and is characterized by hypocalciuria, distinguishing it from primary hyperparathyroidism.

Lithium-Induced Hyperparathyroidism: Lithium therapy can cause hyperparathyroidism by increasing PTH secretion, making it a consideration in patients on long-term lithium treatment.
Vitamin D Deficiency: Although not a direct cause of primary hyperparathyroidism, vitamin D deficiency can lead to secondary hyperparathyroidism, which may be confused with primary hyperparathyroidism in the initial evaluation.
Hypercalcemia of Malignancy: Various malignancies can produce PTH-related protein (PTHrP), leading to hypercalcemia that mimics primary hyperparathyroidism.

Multiple Endocrine Neoplasia (MEN) Syndromes: These are hereditary conditions that can involve multiple endocrine glands, including the parathyroids. MEN syndromes can lead to primary hyperparathyroidism and are critical to diagnose due to their implications for family screening and the potential for other endocrine tumors.
Parathyroid Carcinoma: Although rare, parathyroid carcinoma can present similarly to primary hyperparathyroidism and is crucial to identify due to its different management and prognosis.

Familial Isolated Hyperparathyroidism: A rare genetic condition characterized by hyperparathyroidism without other endocrine abnormalities, often due to mutations in the HRPT2 gene.
Neonatal Severe Primary Hyperparathyroidism: A rare condition presenting in newborns, typically due to homozygous inactivating mutations in the CASR gene, leading to severe hyperparathyroidism.
Parathyroid Hyperplasia due to X-linked Hypophosphatemia: A rare genetic disorder that can lead to parathyroid hyperplasia and hyperparathyroidism as part of its spectrum.