“D2 is a vitamin, D3 is a hormone.”

This is biochemically muddled:

Vitamin D2 (ergocalciferol):

Plant/fungal-derived (e.g., UV-exposed mushrooms, fortified foods).

Vitamin D3 (cholecalciferol):

Produced in human and animal skin from 7-dehydrocholesterol via UV-B. Also used as a common supplement form.

Both D2 and D3 are inactive precursors. They’re hydroxylated to 25-OH-D, then to 1,25-(OH)₂-D (calcitriol), which is the actual hormone that binds the vitamin D receptor.

So:

D2 and D3 are best described as prohormones (and traditionally called “vitamins”).

The hormone is calcitriol, not “D3 itself” as found in supplements.

Functionally, both D2 and D3 end up as the same type of hormone (1,25-dihydroxyvitamin D), though their pharmacokinetics differ.

The real difference between D2 and D3:

Both raise 25-OH-D, but D3 is generally more effective and longer-lasting than D2 at comparable doses.

So the line should NOT be taken literally. A more correct framing:

“Vitamin D (whether D2 or D3) is technically a prohormone; its active metabolite behaves like a hormone. D3 is the human/animal form and is more potent than D2 at raising blood levels.”