It's both isn't? In pill form it's a vitamin... from the sun it's a hormone
The definition of vitamin includes the inability of the organism to synthesize it itself, so vitamin D can’t be an actual vitamin
The definition of vitamin is that it’s gotta be organic, necessary for healthy functioning, and we must be incapable of synthesizing enough of it to survive, even if it’s also available in our food. This is why cholesterol isn’t a vitamin; it’s necessary for us to live, but we’re fully capable of synthesizing enough of it on our own.
Also, nicotinamide (one form of Vitamin B3) is a metabolite of niacin (nicotinic acid, another form of Vitamin B3). Humans can produce the former from the latter, but require at least one of them from the diet. Does that make niacin a vitamin, but nicotinamide a hormone? They are not equivalent, but as far as "being essential", they are substitutes.
(No idea myself, just rumbling thoughts)
I think things get very tricky to define when you need at least one compound that can complete some biological process, but there are several different potential compounds that could do the job, some of which have to come from diet and others can by synthesized by the human body.
Its molecular structure precursor is cholesterol, a steroid hormone which by convention is a molecule with 3 hexanes attached to pentane and an R group, it under goes a series of cleaves in the presence of UV light to yield D3.
D3 and its known analogue's behavioiur and bioactivety also behaves like a steroid/hormone in that it can passively diffuse across a membrane. and helps down-regulate other metabolic reactions. Its effects on reducing inflammation in IG patients is well documented and worth checking out.
Here is a good review on Corticosteroids, specifically on there effects on inflammation treatment.
https://www.medicinenet.com/corticosteroids_vs_nsaids/articl...