@za400 My mom had high blood calcium (hypercalcemia) and it was caused by a problem the parathyroid glands. There are 4 of them in the neck near the thyroid just under the skin. When the bad one was removed, her calcium levels returned to normal. It causes calcium to be removed from bones, and a week after her surgery my mom fell and broke her pelvis, and a bone in her foot.

Here is a Mayo link that discusses this type of issue. They do test blood calcium levels while the patient is on the operating table to determine if the faulty gland has been removed. If levels are still high, they look for another bad one. It was an easy surgery for my elderly mom.
https://www.mayoclinic.org/medical-professionals/endocrinology/news/a-missed-parathyroid-adenoma-a-case-from-the-endocrine-teaching-clinics/mcc-20422649