What is Thyroxine-binding globulin, TBG?

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

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.

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)

References:

Disclaimer:

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