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.
Ectopic secretion
Parathyroid hyperplasia
Parathyroid adenoma
Parathyroid carcinoma