Vitamin D Is Not Just a Vitamin — It’s a Steroid Hormone Scientists confirm that Vitamin D functions as a steroid hormone, not just a traditional vitamin. Unlike most vitamins obtained solely from diet, Vitamin D is synthesized in the skin through sunlight exposure and directly influences gene expression. It regulates calcium balance, immune function, inflammation, and even mood. Important Details: • Produced in skin via sunlight • Structurally classified as a steroid hormone • Regulates calcium