Normal physiological functioning is highly dependent of calcium and the concentration range is very narrow. Normal calcium levels are so crucial to survival that the body will de-mineralize bone if the levels are insufficient. A prerequisite for normal calcium uptake is a normal Vitamin D level. Insufficient levels of Vitamin D are associated to several diseases. The aims of this thesis were to study the relationship between pregnancies and hyperparathyroidism (pHPT) (I), between pHPT and pregnancy with preeclampsia (II) and also to determine if disturbances in calcium homeostasis with vitamin D deficiency are apparent in preeclamptic women (III).  The aim was also to study calciumhomeostasis in obese patients before and after bariatric surgery (IV and V) with emphasis on vitamin D status, parathyroid secretion and bone mineral density (BMD). A correlation was found between a history of pHPT and pregnancy with preeclampsia, with an odds ratio of 6,89 ( 95% CI 2.30, 20.58).  Parathyroid hormone was significantly raised in preeclamptic pregnancies but vitamin D deficiency was present both in preeclamptic and healthy pregnancies. A certain polymorphism of the Vitamin D receptor (baT haplotype), overrepresented in pHPT, was not over expressed in preeclampsia. Hypovitaminosis D was present in more than 70% of bariatric patients preoperatively, which did not change after surgery, despite great weight loss and start of Vitamin D supplementation. BMD was significantly lower in bariatric patients with a negative correlation to the time elapsed since surgery. A small increase in BMD could be noted 10-13 years after bariatric surgery, possibly due to gradual weight gain. CiCa-clamping in obese patients demonstrated a disturbed calcium homeostasis with a left-shifted calcium-PTH relationship and a lower set-point of calcium. This disturbance persisted one year postoperatively. In conclusion, derangements in calcium homeostasis with decreased levels of Vitamin D are present in preeclampsia and obesity. A history of pHPT should be viewed as a risk factor for preeclampsia. Life long follow-up is necessary after bariatric surgery, and an individually adjusted high dose Vitamin D substitute is probably needed to avoid a development of osteoporosis.