The commonest cause of raised serum calcium is -
Primary hyperparathyroidism is the most common cause of hypercalcemia overall.
The frequency of the various parathyroid lesions underlying the hyperfunction is as follows : -

Parathyroid adenoma - 75% to 80%
Primary hyperplasia - 10% to 15%
​Parathyroid carcinoma- < 5%

After hyperparathyroidism, malignancy is the 2nd most common cause of hypercalcemia by one of the following mechanisms.

Osteolysis
Secretion of parathormone related peptide.