It can be synthesized in animal body from cholesterol. Make sure you know all the chemical names involved in this process. Vitamin D deficiency leads to calcicum deficiency. Therefore, deficiency symptoms are the same as those of calcium deficiency. They include abnormal skeletal development and blood clotting problem.
Vitamin D is also quite toxic.
Overdose of vitamin D will cause toxicity. Calcification of soft tissues such as lung, aorta and heart.