vitamin D
Vitamin D is the famous sunshine vitamin, but it is really a hormone in disguise. Your skin makes it when ultraviolet light hits it, and you can also get it from food, but in its raw form it does almost nothing. It must be activated in two steps before it can help you absorb calcium.
After it forms in skin or arrives from the diet, the liver converts it to 25-hydroxyvitamin D, the main storage form measured in blood tests. The kidney then performs the final, tightly controlled step, turning it into calcitriol, the fully active hormone. Calcitriol acts on the gut to absorb calcium and phosphate from food and, with PTH, helps regulate bone. PTH stimulates this kidney activation, linking the two systems.
Deficiency is common in places with little sun, in darker skin, in older adults, and where diets are poor. Without enough vitamin D the gut cannot absorb calcium well, so bones soften, causing rickets in children and osteomalacia in adults. There is also genuine scientific debate about claimed benefits beyond bone, where evidence is far weaker than popular supplements suggest.
A toddler raised indoors with little sun and a milk-free diet develops bowed legs from rickets, the childhood disease of severe vitamin D deficiency.
Rickets is the visible cost of vitamin D deficiency in growing bone.
Vitamin D meets the definition of a hormone (made in one site, activated elsewhere, acting on distant targets). The name vitamin stuck for historical reasons before its hormonal nature was understood.