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Thyroxine-binding globulin (TBG) is a globulin that binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) responsible for carrying the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the bloodstream. Of these three proteins, TBG has the highest affinity for T4 and T3 but is present in the lowest concentration. Despite its low concentration, TBG carries the majority of T4 in the blood plasma.
Thyroxine-binding globulin (TBG) is a globulin that binds thyroid hormones in circulation. It is one of three transport proteins (along with transthyretin and serum albumin) responsible for carrying the thyroid hormones thyroxine (T4) and triiodothyronine (T3) in the bloodstream. Of these three proteins, TBG has the highest affinity for T4 and T3 but is present in the lowest concentration. Despite its low concentration, TBG carries the majority of T4 in the blood plasma.
Interpretation: General
TBG effects measurement of total T4 and T3
Increased TBG increases total T4 and T3 measurements
Decreased percent Free T4 and T3
No change in Free T4 and Free T3
Decreased TBG decreases total T4 and T3 measurements
Increased percent Free T4 and T3
No change in Free T4 and Free T3
Labs unaffected by Thyroid Binding Globulin (TBG):
Free Thyroxine Index (FTI)
Calculation based on T3 Resin Uptake
Free Thyroxine (Free T4)
Free Triiodothyronine (Free T3)
Factors that Increase TBG (Reduces total T4 and T3 measurements)
Pregnancy
Newborn state
Chronic Active Hepatitis
Biliary Cirrhosis
Infection
Acute intermittent porphyria
Familial hyperthyroxinemia
Drugs
Oral Contraceptives
Estrogens
Tamoxifen
Methadone
Heroin
Perphenazine
Factors that decrease TBG (Elevates total T4 and T3 measurements)
Chronic Liver Disease
Obstructive Jaundice or Cirrhosis
Severe systemic illness
Active Acromegaly
Nephrotic Syndrome
Surgical Stress
Opiates
Depakote
Depakene
Dilantin
Phenothiazines
Androgens (testosterone, anabolic steroids)
Salicylates
L-asparaginase
Large doses of glucocorticoids (Prednisone)
