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.
The second reason for a low vitamin D is that you have primary hyperparathyroidism due to a small tumor in your neck and the body converts all the vitamin D-25 (that your doctor is measuring) to the active Vitamin D-1-25 (that doctors do not measure). This is the blue blob on the graph.
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.