Lab

Parathyroid Hormone

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Parathyroid Hormone, 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 in an inverse relationship to Serum Calcium levels
    1. Parathyroid Glands increase PTH secretion with Hypocalcemia (Low Serum Calcium)
    2. Parathyroid Glands decrease PTH secretion with Hypercalcemia (High Serum Calcium)
  2. Pharmacokinetics
    1. PTH is an 84 amino-acid peptide produced and secreted by the Parathyroid Glands
    2. Parathyroid Glands respond to Hypocalcemia or Hypercalcemia within minutes of out-of-range Serum Calcium
    3. Parathyroid Glands hypertrophy in response to sustained Hypocalcemia
    4. Parathyroid production and secretion of PTH
      1. Suppressed by negative feedback from increased Serum Calcium and 1-25 dihydroxyvitamin D
    5. Circulating half-life: 2-5 minutes
    6. 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
  • Labs
  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
  1. Parathyroid Hormone: 10 to 66 pg/ml
    1. Normal Parathyroid Function
  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
  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):333-40 [PubMed]