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Thyroxine-Binding Globulin (TBG)


Definition


  • This glycoprotein is the principal carrier for T3 and T4. It declines with age in parallel with total and free T4 and T3. The latter changes are accompanied by an increase in rT3 and rT3 index, suggesting a decrease in peripheral conversion of T4 and T3 rather than the change in secretory behavior of thyroid gland itself. With the availability of better free thyroid hormone tests, the TBG test is rarely used to assess the thyroid-binding hormone status.
  • Normal range:
    • Male: 1.2 " “2.5 mg/dL
    • Female: 1.4 " “3.0 mg/dL

Use


  • Diagnosis of genetic or idiopathic excess TBG
  • Sometimes used to detect recurrent or metastatic differentiated thyroid carcinoma, especially follicular type and where the patient has had an increased level due to carcinoma
  • To distinguish increased/decreased total T3 or total T4 concentrations due to changes in TBG; same purpose as T3 resin uptake and free thyroxine index

Interpretation


Increased In


  • Pregnancy.
  • Certain drugs (e.g., estrogens, birth control pills, perphenazine, clofibrate, heroin, methadone).
  • Estrogen-producing tumors.
  • Systemic illness is increased early.
  • Acute intermittent porphyria.
  • Acute or chronic active hepatitis.
  • Lymphocytic painless subacute thyroiditis.
  • Neonates.
  • Inherited.
  • Idiopathic.
  • An increased TBG is associated with increased serum T4 and decreased T3 resin uptake; a converse association exists for decreased TBG.

Decreased In


  • Nephrosis and other causes of marked hypoproteinemia such as liver disease, severe illness (late), stress (thyroxine-binding prealbumin [TBPA] also decreased)
  • Deficiency of TBG, genetic or idiopathic
  • Acromegaly (TBPA also decreased)
  • Severe acidosis
  • Certain drugs (e.g., androgens, anabolic steroids; glucocorticoids [TBPA is increased])
  • Testosterone-producing tumors
  • Major illness, surgical stress, protein malnutrition, malabsorption resulting from various causes

Limitations


  • Decreased binding of T3 and T4 due to drugs (salicylates, phenytoin, Orinase, Diabinese, penicillin, heparin, barbital)
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