Is there anything to worry about?
Yes there is. In fact, if you abuse the use of fat burners then there is certainly a lot to be worried about. You would be preparing yourself for side-effects which could be short-term, long-term, reversible or irreversible. Sounds horrifying? It is intended to be so because unless you use these things under proper guidance of an expert or medical practitioner, you are making a good case of what not to do. There have been cases in the past when people go to extremes in their desperation and suffer from side-effects. It is generally not the product that has to be blamed but the greed of the user for overnight success.
The ONLY purpose of the parathyroid glands is to regulate the calcium level in our bodies within a very narrow range so that the nervous and muscular systems can function properly. This is all they do. They measure the amount of calcium in the blood every minute of every day... and if the calcium levels go down a little bit, the parathyroid glands recognize it and make parathyroid hormone (PTH) which goes to the bones and takes some calcium out (makes a withdrawal from the calcium vault) and puts it into the blood. When the calcium in the blood is high enough, then the parathyroids shut down and stop making PTH.