Calcium homeostasis is largely controlled by the parathyroid glands (tucked away underneath the thyroid). I’ve included a little bit of the vitamin D synthesis pathway as well, though D3 (the form that is absorbed in the intestines) is also synthesized in the skin as long as you’re getting a little bit of sunlight.

Hypercalcemia

1. Hyperparathyroidism: usually an adenoma
2. Malignancy: PTH-related peptide released by tumor (squamous cell, renal, breast, bladder)
3. Vitamin D excess: granulomas (sarcoidosis, TB, Wegener’s)
4. Increased bone turnover: hyperthyroidism, Paget’s
5. Familial hypocalcuric hypercalcemia: mutation in the calcium-sensing receptor in parathyroid and kidney

Hypocalcemia

1. Hypoparathyroidism: sporadic, caused by thyroid surgery, Wilson’s, hypomagnesemia
2. Pseudo-Hypoparathyroidism: PTH end-organ resistance
3. Vitamin D deficiency: no sunlight, GI disease
4. Chronic renal failure: decreased 1,25(OH)D production, increased phosphate
5. Calcium sequestration: acute phosphate increase