D₃ (cholecalciferol) from exposure of skin (stratum basale) to sun, ingestion of fish, milk, plants.D₂ (ergocalciferol) from ingestion of plants, fungi, yeasts.Both converted to 25-OH D₃ (storage form) in liver and to the active form 1,25-(OH)₂ D₃ (calcitriol) in kidney.

- ↑ intestinal absorption of Ca²⁺ and PO₄^3-- ↑ bone mineralization- ↑ bone resorption at higher levels- ↑ PTH → ↑ Ca²⁺ reabsorption and ↓ PO₄^3- reabsorption in the kidney.

Deficiency:- Rickets in children (bone deformity, such as genu varum "bow legs")- Osteomalacia in adults (bone pain and muscle weakness)- Hypocalcemic tetany- Caused by malabsorption, ↓ sun exposure, poor diet, chronic kidney disease

- Deficiency is exacerbated by pigmented skin, premature birth.- Give oral vitamin D to breastfed infants.

Excess: Hypercalcemia, hypercalciuria, loss of appetite, stupor. Seen in granulomatous disease (↑ activation of vitamin D by epithelioid macrophages).