Parathyroid Hormone

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

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II. Physiology: Parathyroid Hormone (PTH)

  1. Images
    1. calciumHomeostasis.png
  2. Anatomy
    1. Four small Parathyroid Glands are found at the posterior aspect of Thyroid Gland
  3. 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)
  4. 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
  5. Mechanism
    1. See Calcium Homeostasis
    2. PTH stimulates Osteoclasts in bone
      1. Osteoclasts mobilize bone Calcium
      2. Phosphates are also mobilized by bone, but with increased renal phosphate excretion
    3. 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 Calcium and phosphate via intestinal and renal absorption and bone mobilization

III. 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

IV. Interpretation: Normal Serum Calcium

  1. Parathyroid Hormone: 10 to 66 pg/ml
    1. Normal Parathyroid Function

V. 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

VI. 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

VII. 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]

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