Hypocalcemia may be classified functionally into complete absence of parathyroid gland, parathyroid hormone (PTH) insufficiency and PTH overactivity.

• Hypocalcemia may be classified functionally into the following sub-types:

• Absence of PTH is primarily seen in^{[1][2][3][4][5]}
  • Hereditary hypoparathyroidism
  • Acquired hypoparathyroidism (e.g. surgical resection)
  • Hypomagnesemia

• PTH insufficiency is seen when the active form of vitamin D (1,25 di-Hydroxy-Vitamin D or Vitamin D3) is lacking. This may be seen in^[6][7]
  • Reduced dietary intake of vitamin D
  • Insufficient exposure to sunlight
  • Vitamin D-dependent rickets type 1
  • Chronic renal failure
• PTH insufficiency is seen in active vitamin D ineffectiveness which is seen in
  • Intestinal malabsorption
  • Vitamin D-dependent rickets type 2
  • Pseudohypoparathyroidism

• PTH overactivity is seen in^[8][9]
  • Severe hypophosphatemia
  • Tumor lysis syndrome
  • Acute renal failure
  • Rhabdomyolysis