Vitamin D Is Not A Vitamin But A Steroid Hormone Precursor
(or so I just found by googling)
A hormone is a blood-borne chemical secreted by an endocrine gland and which affects the physiology of organ(s) in other parts of the body.
Biologically-active vitamin D is actually produced via the liver and the kidney. The vitamin D produced by the skin does not have biological activity until it is actrivated by the liver and kidney. So, I wouldn't call it a hormone, but a precursor, in much the same manner that cholesterol is not a hormone but a precursor for steroids made in the ovary, testes, adrenal, etc.