The simple view of the bonding in methane You will be familiar with drawing methane using dots and crosses diagrams, but it is worth looking at its structure a bit more closely. There is a serious mis-match between this structure and the modern electronic structure of carbon, 1s 2 2s 2 2p x 1 2p y 1.
Lack of sun exposure: Lack of sun exposure is a major cause of low vitamin d level especially if you live in the northern us. Less likely would be lack of vitamin d in food, but also a possibility if you are on a very restrictive diet.
They think that the low vitamin D levels cause too little calcium to be absorbed in the intestines. They think that this low amount of calcium is sensed by normal parathyroid glands which causes the normal parathyroid glands to appropriately increase their production... causing a high PTH level.