Primary Hyperparathyroidism: This condition is characterized by the excessive production of parathyroid hormone (PTH), leading to increased calcium levels in the blood. The elevated PTH also increases phosphate excretion in the urine, but in the context of very high PTH levels, phosphate can initially be elevated due to increased bone resorption.

Malignancy-associated Hypercalcemia: Certain types of cancer, such as multiple myeloma, lung cancer, and breast cancer, can lead to elevated calcium levels through various mechanisms, including the production of PTH-related protein (PTHrP) or osteolytic lesions. Phosphate levels can be variable but may be elevated in the setting of tumor lysis syndrome or bone metastasis.
Vitamin D Intoxication: Excessive intake of vitamin D can lead to increased absorption of calcium from the gut, resulting in hypercalcemia. Vitamin D also increases phosphate absorption, potentially leading to elevated phosphate levels.
Familial Hypocalciuric Hypercalcemia (FHH): A genetic disorder characterized by an inborn error of the calcium-sensing receptor, leading to inappropriate secretion of PTH and consequent hypercalcemia. Phosphate levels can be elevated due to the increased PTH.

Hypercalcemia of Malignancy with PTHrP: While included under malignancy-associated hypercalcemia, it's crucial to identify this specific cause due to its implications for cancer diagnosis and treatment. PTHrP can mimic the action of PTH, leading to elevated calcium and variable phosphate levels.
Sarcoidosis: This granulomatous disease can lead to elevated vitamin D levels, causing increased absorption of calcium and potentially phosphate. It's a critical diagnosis not to miss due to its systemic implications and the availability of specific treatments.

Hyperthyroidism: Although more commonly associated with hypercalcemia due to increased bone turnover, severe hyperthyroidism can also lead to elevated phosphate levels in some cases, particularly if there's significant bone resorption.
Pseudohypoparathyroidism: A rare genetic disorder characterized by resistance to PTH, which can lead to elevated phosphate levels. However, calcium levels are typically low, making this a less likely cause of raised calcium and phosphate.
Lithium Therapy: Long-term lithium use can lead to hyperparathyroidism, resulting in elevated calcium and potentially phosphate levels due to the increased PTH secretion.