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Parathyroid HormoneAka: C-Terminal PTH, N-Terminal PTH, Whole Molecule PTH, Intact PTH, Mid-Region PTH, PTH

Physiology: Parathyroid Hormone (PTH)
1. PTH is released by Parathyroid Glands in neck
  1. Parathyroids increase PTH when calcium is low
  2. Parathyroids decrease PTH when calcium is high
2. Pharmacokinetics
  1. PTH is 84 amino-acid peptide
  2. Circulating half-life: 2-5 minutes
  3. Metabolized by liver and kidney
3. Mechanism
  1. PTH stimulates Osteoclasts in bone
    1. Osteoclasts mobilize bone calcium
  2. PTH acts at kidneys to increase Serum Calcium
    1. Increases calcium reabsorption
    2. Kidney activates 25-hydroxyVitamin D
      1. Converts it to 1,25-dihydroxyVitamin D3
      2. Increases intestinal calcium absorption
Testing
1. Tests to avoid
  1. C-Terminal PTH
  2. N-Terminal PTH
2. Recommended Test
  1. Whole Molecule PTH by IRMA (Mid-region or Intact PTH)
    1. Measures C-Terminal and N-Terminal PTH
Interpretation: Normal Serum Calcium
1. Parathyroid Hormone: 10 to 66 pg/ml
  1. Normal Parathyroid Function
Interpretation: High Serum Calcium
1. Parathyroid Hormone >66 pg/ml
  1. Primary Hyperparathyroidism
  2. Consider MEN I or MEN II
2. Parathyroid Hormone 10-66 pg/ml
  1. Familial Hypocalciuric Hypercalcemia
    1. 24 hour Urine Calcium to Creatine clearance <0.01
3. Parathyroid Hormone <10 pg/ml
  1. ParaThyroid Cancer
  2. Granulomatous Disease
  3. Milk-Alkali Syndrome
  4. Adrenal Insufficiency
  5. Hyperthyroidism
  6. Prolonged immobilization
  7. Medication related
    1. Lithium
    2. Theophylline
    3. Tamoxifen
Interpretation: Low Serum Calcium
1. Parathyroid Hormone >66 pg/ml
  1. Secondary Hyperparathyroidism
    1. Renal Failure
    2. Gastrointestinal malabsorption
    3. Hypomagnesemia
2. Parathyroid Hormone 10 to 66 pg/ml
  1. PseudoHypoparathyroidism
3. Parathyroid Hormone <10 pg/ml
  1. Hypoparathyroidism
References
1. Bakerman (1984) ABCs of Lab Data, ILD, Greenville, NC
2. Einhorn (2001) CMEA Medicine Lecture, San Diego
3. Taniegra (2004) Am Fam Physician 69(2):333

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