How serious is it to have low vitamin d levels? My blood test showed 18 vit d, but my doc said to ignore that bc calcium level is normal. True?

Take the D. You need at least 2000 units of vitamin d3/day and recheck your levels in 3 mos. Or so. They are not dangerous, but need to be improved. Also make sure your thyroid level is normal and that you are receiving brand name T4 replacement, as the generics are unreliable.
Misunderstood? Low vitamin d causes poor calcification of the bones and is likely to be painful long before it makes the serum calcium abnormal. Please supplement and/or get sunlight.
Disagree. Institute on medicine says you should be above 30. Some think that some cancers are associated with chronic low vitamin D. 40% of North Americans are low. Middleton L, Stack BC Jr, Riggs AT, Bodenner DL. Vitamin D deficiency: asimple algorithm employing weekly administration of 50,000 IU of vitamin D. Am J Otolaryngol. 2014 Mar-Apr;35(2):85-8. doi: 10.1016/j.amjoto.2013.12.002. Epub 2013 Dec 1.
Unfortunately. No! a level of 18 is disastrous since it needs to be between 60 and 80. Vit d is made in the skin by exposure to sunlight. We now know that it is needed by every cell in the body. It affects everything, not just bone metabolism. If taking a supplement it should be a huge dose like 10000 units a day for several weeks or months and then get it checked again. There is a lot of new info about d.