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.
As he mentioned towards the end of his passage, one of the differences he states is, “1. Signs are what a doctor sees, symptoms are what a patient experiences.” if i might add that when he means what a doctor “sees” also means what is smelt, felt, tasted(in rare cases of diagnosis), hear, and sees or the 5 senses. This is the difference that he is talking about when differentiating between a sign and a symptom. A simpler way to word this is that it is all observed by perspective. A fever can be felt and seen by a doctor making it a sign but it can also be a symptom do to the fact that the patient also experiences the fever. So you are right, but technically it is not a special case because there are also other symptoms/signs that fall under this category.