Hi.. Vitamin D is secreted by skin when exposed to sunlight.. Vitamin D effects absorption of calcium and mineralisation of bone resulting in lowering of the serum calcium levels.. These effects are the shared by the parathormone secreted by the parathyroid glands...
Removal of the parathyroid glands leads to deficiency of this hormone and thereby the responsibilities of bone mineralisation is increased on the vitamin D and hence the vitamin D supplementation..
This is the basis of vitamin D supplementation and not entirely due to increased calcium.. Hope its clear now.. All the best...