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.
- Adult Male: 12.7-25.1 mcg/mL
- Adult Female 13.5-30.9 mcg/mL
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
Note: TBG levels are normally high in newborns.
Decreased TBG levels may be due to:
- Acute illness
- Chronic liver disease
- Acromegaly (disorder caused by too much growth hormone)
- Hyperthyroidism (overactive thyroid)
- Nephrotic syndrome (symptoms that show kidney damage is present)
- Stress from surgery
- Large doses of glucocorticoids (Prednison)
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