Thyroxine Binding Globulin

Thyroxine‑binding globulin (TBG) is the primary plasma transport protein for thyroid hormones, binding most circulating T4 and T3. By sequestering hormones it regulates the free (bioactive) hormone fraction, stabilizes levels and prolongs hormone half‑life, acting as a circulating reservoir. TBG concentration and binding capacity influence tissue hormone availability and are altered by pregnancy, estrogens, liver disease, and genetic variants.

High TBG raises total T4 and T3 measurements while free hormone levels and TSH usually stay normal, so patients remain clinically euthyroid. Causes include pregnancy, estrogen therapy, and hereditary TBG excess. Elevated TBG is usually a laboratory effect that doesn’t require treatment; clinicians use free thyroid hormone or TSH testing to avoid misdiagnosis of hyperthyroidism.

Thyroxine-binding globulin (TBG) primarily binds T4 with high affinity and also binds T3 but with lower affinity. It carries the majority of circulating thyroid hormones—especially T4—while albumin and transthyretin bind smaller fractions. Changes in TBG alter total thyroid hormone levels but generally do not change free (active) T3 or T4 concentrations.

Normal serum thyroxine‑binding globulin (TBG) is typically about 14–31 µg/mL (equivalent to 14–31 mg/L), though reference intervals vary by laboratory, assay, sex and pregnancy. TBG is raised by estrogen, pregnancy and some liver conditions, and reduced by androgens, nephrotic syndrome and hyperthyroidism. Always compare your result to the reference range reported by the testing lab.

Low TBG occurs when synthesis falls or the protein is lost. Common causes include chronic liver disease (reduced production), nephrotic syndrome or protein‑losing enteropathy (urinary/enteric loss), severe illness or malnutrition, certain drugs and hormones (androgens, anabolic steroids, high‑dose glucocorticoids), and rare congenital TBG deficiency. Low TBG lowers total thyroid hormone levels but typically won’t change free hormone concentrations.

Low T4 (low thyroxine) means your thyroid isn’t making enough hormone, indicating hypothyroidism. Symptoms can include fatigue, weight gain, cold intolerance, constipation, dry skin, slow heart rate, and menstrual changes. Causes include autoimmune thyroiditis, iodine deficiency, surgery, some medications, or pituitary problems. Doctors use TSH and free T4 tests to confirm and may treat with levothyroxine if needed.