1. Daniel Bikle, John Adams, and Sylvia Christakos
Chapter 28: Vitamin D: Production, Metabolism, Mechanism of Action, and Clinical Requirements
Daniel Bikle, John Adams, and Sylvia Christakos

2. From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7th Edition.
Figure 1
Figure 1 The photolysis of ergosterol and 7-dehydrocholesterol to vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), respectively. An intermediate is formed after photolysis, which undergoes a thermal-activated isomerization to the final form of vitamin D. The rotation of the A-ring puts the 3β-hydroxyl group into a different orientation with respect to the plane of the A-ring during production of vitamin D.
Figure 1 The photolysis of ergosterol and 7-dehydrocholesterol to vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol), respectively. An intermediate is formed after photolysis, which undergoes a thermal-activated isomerization to the final form of vitamin D. The rotation of the A-ring puts the 3β-hydroxyl group into a different orientation with respect to the plane of the A-ring during production of vitamin D.
© 2008 American Society for Bone and Mineral Research

3. From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7th Edition.
Figure 2
Figure 2 The metabolism of vitamin D. The liver converts vitamin D to 25OHD. The kidney converts 25OHD to 1,25(OH)2D3 and 24,25(OH)2D3. Control of metabolism is exerted primarily at the level of the kidney, where low serum phosphorus, low serum calcium, low FGF23, and high parathyroid hormone (PTH) levels favor production of 1,25(OH)2D3, whereas high serum phosphorus, calcium, FGF23, and 1,25(OH)2D3 and low PTH favor 24,25(OH)2D3 production.
Figure 2 The metabolism of vitamin D. The liver converts vitamin D to 25OHD. The kidney converts 25OHD to 1,25(OH)2D3 and 24,25(OH)2D3. Control of metabolism is exerted primarily at the level of the kidney, where low serum phosphorus, low serum calcium, low FGF23, and high parathyroid hormone (PTH) levels favor production of 1,25(OH)2D3, whereas high serum phosphorus, calcium, FGF23, and 1,25(OH)2D3 and low PTH favor 24,25(OH)2D3 production.
© 2008 American Society for Bone and Mineral Research

4. From the Primer on the Metabolic Bone Diseases and Disorders of Mineral Metabolism, 7th Edition.
Figure 3
Figure 3 1,25(OH)2D3 interacts with other hormones, in particular FGF23 and PTH, to regulate calcium and phosphate homeostasis. As noted in the legend to figure 2, FGF23 inhibits whereas PTH stimulates 1,25(OH)2D3 production by the kidney. In turn 1,25(OH)2D3 inhibits PTH production but stimulates that of FGF23. Calcium and phosphate in turn regulate FGF23, PTH, and so 1,25(OH)2D3 indirectly.
Figure 3 1,25(OH)2D3 interacts with other hormones, in particular FGF23 and PTH, to regulate calcium and phosphate homeostasis. As noted in the legend to figure 2, FGF23 inhibits whereas PTH stimulates 1,25(OH)2D3 production by the kidney. In turn 1,25(OH)2D3 inhibits PTH production but stimulates that of FGF23. Calcium and phosphate in turn regulate FGF23, PTH, and so 1,25(OH)2D3 indirectly.
© 2008 American Society for Bone and Mineral Research