Lab

Thyroxine-Binding Globulin

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Thyroxine-Binding Globulin, Thyroxine Binding Globulin, Thyroid Binding Globulin, TBG

  • Physiology
  1. Radioimmunoassay of major storage of Thyroid hormone
  2. TBG binds Thyroid hormone in circulation
    1. Thyroxine (T4) is 99.97% protein bound
    2. Triiodothyronine (T3) is 99.7% protein bound
  • Interpretation
  • General
  1. TBG effects measurement of total T4 and T3
    1. Increased TBG increases total T4 and T3 measurements
      1. Decreased percent Free T4 and T3
      2. No change in Free T4 and Free T3
    2. Decreased TBG decreases total T4 and T3 measurements
      1. Increased percent Free T4 and T3
      2. No change in Free T4 and Free T3
  2. Labs unaffected by Thyroid Binding Globulin (TBG)
    1. Free Thyroxine Index (FTI)
      1. Calculation based on T3 Resin Uptake
    2. Free Thyroxine (Free T4)
    3. Free Triiodothyronine (Free T3)
  • Interpretation
  • Increased TBG (Reduces total T4 and T3 measurements)
  1. Pregnancy
  2. Newborn state
  3. Chronic Active Hepatitis
  4. Biliary Cirrhosis
  5. Infection
  6. Acute intermittent porphyria
  7. Familial hyperthyroxinemia
  8. Drugs
    1. Oral Contraceptives
    2. Estrogens
    3. Tamoxifen
    4. Methadone
    5. Heroin
    6. Perphenazine
  • Interpretation
  • Decreased TBG (Elevates total T4 and T3 measurements)
  1. Chronic Liver Disease
  2. Obstructive Jaundice or Cirrhosis
  3. Severe systemic illness
  4. Active Acromegaly
  5. Nephrotic Syndrome
  6. Surgical Stress
  7. Medications
    1. Androgens
    2. Salicylates
    3. L-asparaginase
    4. Large doses of glucocorticoids (Prednisone)