PTH and hyperphosphatemia: Phosphate and vitamin D also modulate PTH levels: hyperphosphatemia (even independent of calcium changes) stimulates PTH secretion, partly via a direct inhibitory effect of high phosphate on CaSR [27]; whereas 1,25-dihydroxyvitamin D (calcitriol) acts on vitamin D receptors (VDRs) in parathyroid cells to repress PTH gene transcription [25].