There are two forms of calcium that can be detected by blood draw: serum calcium and ionized calcium. One binds with the proteins in blood, the other is free of the proteins. That's about all I really remember from all the reading I was doing (mostly on osteomalacia, it relationship (if any) to osteoporosis) and hyperparathyroidism. There are also two forms of vitamin D that can be tested by blood draw: vitamin D25 hydroxy (calcidiol) and vitamin D1 25 hydroxy (calcitriol) and the results of one is more indicative of your true state of vitamin D level than the other. When my former Endocrinologist suspected that I had osteomalacia as well as osteoporosis and secondary hypoparathyroidism the blood draws she initiated were the Parathyroid hormone intact with ionized calcium, vitamin D 25 hydroxy and serum calcium. Prior to starting me on Forteo (or even IV pamidronate) she wanted my PTH level below 30 (normal reference range 10-65) and my vitamin D 25 hydroxy level (reference range 10-68) higher but she never said how high she wanted the D 25 hydroxy (mine was running 32 at the time) nor why. She is an excellent Endo who really knows her stuff, is great with patients in the office with answering questions, patience, etc. - BUT - her desk dragons were a disaster and there was no getting answers to questions, etc. except when you saw her in person. I wasn't the only one who had this problem and have since talked to others who all agree she was wonderful but her office was a disaster and they've changed Endos as well. Questioning your doctor until you understand is best, but internet searches (watch for the HON code seal) and PubMed searches can be a great help in understanding and in contemplating your options.