Vitamin D is unique as it functions as a vitamin and a hormone. When the skin is exposed to sunlight, it synthesizes vitamin D, and the active form of vitamin D acts as a hormone, playing a crucial role in calcium metabolism, bone health, and other cellular processes.
Vitamin D is classified as a fat-soluble vitamin, specifically vitamin D3 (cholecalciferol) which is the form synthesized in the skin upon exposure to sunlight. Calcitriol is a type of vitamin D that acts as a hormone in the body.