PTH and CaSR Receptor Kinetics

Parathyroid hormone (PTH) is an 84-amino acid peptide that maintains serum ionized calcium within a tight physiological range through an inverse sigmoidal feedback loop governed by the calcium-sensing receptor (CaSR) on chief cells. Secretion operates across three temporal scales: immediate exocytosis of preformed granules, intermediate upregulation of PTH mRNA transcription, and chronic glandular hyperplasia. Active 1,25-dihydroxyvitamin D directly suppresses PTH synthesis, whereas indirect stimulation occurs via elevated serum phosphate. Crucially, severe hypomagnesemia paradoxically impairs PTH secretion by blunting stimulus-secretion coupling, creating functional hypoparathyroidism.