Parathyroid Hormone

Normal range 10-55pg/ml

(PTH)

• controls calcium and phosphate levels

Production

• 84 amino acid polypeptide
• produced in chief cell of parathyroid gland

Control

• secreted in response to decrease in ionised calcium
• PTH release suppressed by
  • 1,25-(OH)₂D
• cellular mechanism
  • as calcium levels rise,
  • cell surface receptor causes decreased c.AMP through inhibitory G-protein
  • lowered c.AMP and raised intracellular calcium suppress PTH release
• other factors
  • weakly controlled by levels of magnesium
  • PTH release stimulated by
    • adrenaline through b receptors
    • glucocorticoids

Actions

• In bone
  • stimulates osteocytes to release calcium and phosphate into circulation (reaction occurs within minutes)
  • causes osteoclasts to differentiate from precursors and become active in bone resorption
• In kidney
  • stimulates 1,25-(OH)₂D production
  • decreases proximal tubule reabsorption of phosphate (increased excretion)
  • increase distal nephron reabsorption of calcium
  • decreases bicarbonate reabsorption

Measuring

• plain (white top) tube, 5mls
• needs to be taken to lab immediately

Disorders

• primary hyperparathyroidism
• secondary hyperparathyroidism
• tertiary hyperparathyroidism
  • may be associated with hyperuricaemia
  • development of large cavities in bone through increased reabsorption
  • increased calcium in urine due to increased filtered calcium load
• hypoparathyroidism
  • also leads to increased urinary secretion of calcium due to removal of inhibitory effects on distal tubule
• ectopic parathyroid hormone / parathyroid hormone related product production

14.7.1 PTH = parathyroid hormone — principal control of plasma calcium

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