The parathyroids differ from other endocrine glands in several respects. (1) The parathyroids appear late in phylogeny, being seen first in the amphibia. (2) While most endocrine glands make more than one hormone, only one hormone and one secretory cell type are regularly found in the parathyroids. (3) While most endocrine glands are under complex control involving both long and short feedback loops, the parathyroid glands have a unique feedback control by the concentration of divalent cations in the blood. Of course, indirect effects on parathyroid function can occur which act via changes in divalent cation concentration, so that the complexity of parathyroid control may be as great as that of other endocrines.
The physiologic role of the parathyroid glands is to regulate serum ionized calcium concentration. However, changes in the concentration of both magnesium and calcium ions can affect parathyroid function. The chief reason that the parathyroids regulate