<prompt> Answer this question truthfullyHow does a severe decrease in serum Mg2 concentration affect the secretion of PTH, and what is the role of PTH in the body?  <response>: <reason> A severe decrease in serum Mg2 concentration causes a decrease in PTH secretion. PTH parathyroid hormone is a hormone that regulates calcium and phosphate homeostasis in the body. When magnesium levels are low, PTH secretion decreases, which can lead to hypocalcemia low calcium levels and other related disorders. 