Thyroxine Binding Globulin

Blood
Optimal Result: 13 - 39 ug/ml.

Thyroid-binding globulin (TBG) is produced in the liver and is the primary circulating (transport) protein that binds thyroid hormones3,5,3’-triiodothyronine (T3) and thyroxine (T4) and carries them in the bloodstream (think of it like a taxi cab that shuttles T3 and T4 around the body).

There are 2 other thyroid hormone transport proteins (Transthyretin & serum Albumin), but TBG has the highest affinity (=the degree to which a substance tends to combine with another) for T4 and T3 but is present in the lowest concentration.

Reference Ranges:

  • Adult Male: 12.7-25.1 mcg/mL
  • Adult Female 13.5-30.9 mcg/mL

References:

  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2417873/
  • https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/thyroxine-binding-globulin

What does it mean if your Thyroxine Binding Globulin result is too low?

Decreased TBG levels may be due to:

  • Acute illness
  • Chronic liver disease
  • Acromegaly (disorder caused by too much growth hormone)
  • Hyperthyroidism (overactive thyroid)
  • Malnutrition
  • Nephrotic syndrome (symptoms that show kidney damage is present)
  • Stress from surgery
  • Medications:
    • Androgens
    • Salicylates
    • L-asparaginase
    • Large doses of glucocorticoids (Prednison)

What does it mean if your Thyroxine Binding Globulin result is too high?

An increased TBG level may be due to:

  • Acute intermittent porphyria (a rare metabolic disorder)
  • Hypothyroidism (underactive thyroid)
  • Liver disease
  • Pregnancy (TBG levels normally increased during pregnancy)
  • Drugs that can cause high TBG levels:
    • Oral Contraceptives
    • Estrogens
    • Tamoxifen
    • Methadone
    • Heroin
    • Perphenazine

Note: TBG levels are normally high in newborns.

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