Figure 1. Causes and metabolism of vitamin D intoxication: Increased intake of vitamin D by causes such as vitamin D poisoning, iatrogenic, or accidental overdose raises the levels of serum inactive 25(OH)D. Inactive 25(OH)D (25-hydroxyvitamin D) is then converted to active 1,25(OH)2D (1,25-dihydroxyvitamin D) by 1α-hydroxylase in the kidney. Vitamin D toxicity can also occur via excessive production of active 1,25(OH)2D by causes such as granulomatous disorders or idiopathic infantile hypercalcemia (IIH). 1,25(OH)2D has a high affinity for vitamin D receptors (VDRs) resulting in increased gene expression of target cells. Catabolism of 1,25(OH)2D by 24-hydroxylase generates inactive 24,25-(OH)D (24-hydroxylase).