Parathyroid hormone is responsible for regulating what?

Parathyroid Hormone Regulation

Parathyroid hormone (PTH) is responsible for regulating serum calcium and phosphate concentrations through coordinated actions on the kidneys, bones, intestines, and vitamin D metabolism. 1

Primary Regulatory Functions

Calcium Homeostasis

PTH serves as the central regulator of extracellular calcium through three primary mechanisms:

• Renal calcium reabsorption: PTH increases calcium absorption in both the distal and proximal renal tubules, reducing urinary calcium losses 1, 2
• Bone calcium mobilization: PTH stimulates bone resorption through PTH1R activation, releasing calcium and phosphate from bone into circulation 1, 3
• Intestinal calcium absorption: PTH indirectly enhances intestinal calcium absorption by stimulating renal 1α-hydroxylase (CYP27B1), which converts 25-hydroxyvitamin D to the active form 1,25-dihydroxyvitamin D 1

Phosphate Regulation

PTH exerts critical phosphaturic effects that maintain phosphate balance:

• Renal phosphate excretion: PTH binding to PTH1R in the proximal renal tubule decreases phosphate reabsorption, promoting phosphate excretion 1, 4
• Compensatory mechanism: The phosphaturic effect becomes increasingly important as kidney function declines, particularly in chronic kidney disease 4
• Net effect on phosphate: While PTH releases phosphate from bone, its renal phosphaturic action prevents net phosphate accumulation, maintaining normal serum phosphate levels 1

Integrated Hormonal System

PTH functions within a complex regulatory network involving three central hormones:

• PTH, 1,25-dihydroxyvitamin D, and FGF23 work together to maintain calcium and phosphate homeostasis through actions on bone, kidney, intestine, and parathyroid glands 1, 4
• Calcium-sensing receptor: Hypocalcemia is detected via the calcium-sensing receptor on parathyroid glands, triggering PTH release within seconds 1
• Feedback mechanisms: PTH secretion is regulated by serum calcium levels, vitamin D metabolites, and serum phosphorus 1

Clinical Pitfalls

Assay Variability

• Different PTH assay generations (second vs. third generation) can yield measurements varying by up to 47%, affecting clinical decision-making 1, 4
• Second-generation "intact PTH" assays overestimate biologically active PTH by detecting inactive fragments like 7-84 PTH 4
• Sequential measurements should always use the same assay in the same laboratory to ensure accurate trend evaluation 4

Chronic Kidney Disease Considerations

• In CKD, attempting to maintain PTH in the "normal" range (below 65 pg/mL) can lead to adynamic bone disease; stage-specific targets should be used instead 4
• When creatinine clearance falls below 20-30 mL/min/1.73 m² (CKD Stage 4), the maximum phosphaturic effect of PTH is reached, and serum phosphorus begins to rise despite elevated PTH 4