Parathyroid hormone (PTH) increases osteolysis, renal tubular resorption of calcium, and renal synthesis of 1,25-dihydroxycholecalciferol AND inhibits renal tubular reabsorption of phosphate and bicarbonate, leading to net INCREASE in serum calcium
Hypoparathyroid thus results in hypocalcemia with hyperphosphatemia
Iatrogenic:
Anterior neck surgery or radiation (e.g. resection of multiple parathyroid adenomas, thyroidectomy); may be transient
steroids, diuretics, antiepilptics
Congenital (e.g. DiGeorge syndrome)
Malignancy
Autoimmune
Damage from Heavy metals (e.g. copper in Wilson's disease)
Functional hypoPTH from hypomagnesemia
Anterior neck surgery or radiation (e.g. resection of multiple parathyroid adenomas, thyroidectomy); may be transient
steroids, diuretics, antiepilptics
Hypoparathyroidism
Vitamin D deficiency
Hypothyroidism
Pseudohypoparathyroidism 
Congenital adrenal hyperplasia
Kidney disease
Levothyroxine toxicity
Malabsorption