• Vitamin D deficiency due to inadequate synthesis, intake or absorption • Increased phosphate load:    ∘ Enteral feeding with cow's milk    ∘ Parenteral nutrition • Primary Hypoparathyroidism:    ∘ Isolated vs. Syndromic • Secondary Hypoparathyroidism:    ∘ Pseudohypoparathyroidism due to renal disease or GNAS mutations    ∘ Congenital heart disease    ∘ Renal disease    ∘ Gastrointestinal disease    ∘ Critical illness • Osteopetrosis. Here, GNAS is linked to osteopetrosis.