Does a sudden drop in vitamin d levels mean parathyroid problems?

No: There is an inverse relationship between PTH and vitamin d meaning that once vitamin d (called 25 hydroxy vitamin d) drops below a certain level, ipth starts to rise as a compensatory adaptation to ensure adequate calcium retention from the kidneys is made. So there is no parathyroid problem per say but just an appropriate physiological rise.Chronic PTH elevation is known to be deleterious to bone.

No: Vitamin d absorption depends on a lot of things. It is also affected by sun exposure. I always tell my patients to take additional supplement in the winter time when the sun may not be as strong. Also other things may interfere with the absorption. But i fo not feel it is indicative of a parathyroid problem.