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.
Low vitamin D can result in elevation of the PTH (parathyroid hormone). When the vitamin D is low, it can result in inadequate absorption of calcium and when that occurs, the parathyroid takes action to make sure there is adequate calcium in your blood. So, it is important to have adequate vitamin D, as well as adequate calcium, daily.