Metabolic activation of vitamin D3 to its hormonal form, 1,25(OH)2D3.

Vitamin D is recognized as a prohormone. There are different types of vitamin D, including fat-soluble secosteroids, i.e., vitamin D1-D5. In humans, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) are most important for biological functions. Vitamin D is mostly synthesized in the skin in the presence of sunlight, while some, particularly vitamin D2 and D3, are taken from food sources.

When human skin is exposed to sunlight, 7-dehydrocholesterol is transformed into D3; subsequently, it is converted to 25-hydroxyvitamin D (25(OH)D, also known as calcidiol or calcifediol, in the liver. The active form of vitamin D3, which is also known as calcitriol, 1α,25-dihydroxy vitamin D3, or 1,25-dihydroxycholecalciferol, is synthesized in the kidney in the presence of the enzyme 1α-hydroxylase. Vitamin D levels are determined by estimation of serum 25(OH)D. The Role of Vitamin D in Hormonal Balance (opens in a new tab)

Vitamin D and parathyroid hormone (PTH) are involved in the regulation of calcium and bone formation. Vitamin D elevates calcium absorption from the small intestine, inhibits PTH, and decreases calcium excretion in the kidneys. This ultimately increases calcium concentration in the blood and promotes bone formation, growth, and maintenance. PTH and Vitamin D (opens in a new tab)

Vitamin D Analogues D2 D2-3 Sulfate D3 D3-3 sulfate D4 D5 D6 D7

25(OH)D2 25(OH)D3 25(OH)D4 25(OH)D2 sulfate 25(OH)D3 sulfate 25(OH)D4 sulfate Epi-3-25(OH)D2 Epi-3-25(OH)D3 Epi-3-25(OH)D4 Epi-3-25(OH)D2 Sulfate Epi-3-25(OH)D3 Sulfate Epi-3-25(OH)D4 Sulfate 25(OH)D3-Lactone 25(OH)D3 Glucuronide 25(OH)L3

20(OH)D3 20S(OH)D3 22(OH)D3 20,22(OH)2D3 20,23(OH)2D3 20,24(OH)2D3 20,25(OH)2D3 20,26(OH)2D3 17,20,23(OH)3D3 1,20(OH)2D3 1,20,23(OH)3D3 1,20,22(OH)3D3 20,24(OH)2D3 1,20,24(OH)3D3 1,20,25/OH)3D3 1,20,26(OH)3D3 20,25(OH)2D3 1,20,25(OH)3D3 20,26(OH)2D3 1,20,26(OH)3D3

1α,25(OH)2-3-Epi-Vitamin D3

1a,25(OH)2D2 (alpha) 1a,25(OH)2D3 (alpha) 1b,25(OH)2D3 (beta) 23S,25(OH)2D3 24R,25(OH)2D3 25,26(OH)2D3 4a,25(OH)2D3 (alpha) 4b,25(OH)2D3 (beta) 1,23,25(OH)3D3 1,24,25(OH)3D3

Lumisterol3 (L3) 20(OH)L3 22(OH)L3 20,22(OH)2D3 Pregnalumisterol (pL) 17-hydroxypregnalumisterol 17,20dihydroxypregnalumisterol 5,6 Trans-Vitamin D Tachysterol Toxisterol Suprasterol l Suprasterol ll

20(OH)-7DHC

There's even more pathways for the indole system. That occurs afterwords