Thyroid Biomarkers: Thyroid Binding Globulin (TBG)

Optimal Takeaways

Thyroid binding globulin (TBG) is the primary carrier of T3 and T4 in the blood, serving as a transporter and a stabilizing factor for thyroid hormones. Levels can be decreased with hyperthyroidism, malnutrition, low protein levels, increased protein losses via the kidney, significant stress, steroids, and other medications. Levels can be increased with hypothyroidism, phthalate exposure, hormone therapy, hepatitis, and certain medications.

Standard Range: 11.00 – 31.00 mcg/mL (141.90 – 399.90 nmol/L)    

The ODX Range: 11.00 – 21.00 mcg/mL (141.90 – 270.90 nmol/L

Low TBG is associated with acquired and inherited TBG deficiency (Chakravarthy 2022), hyperthyroidism (Azad 2011), hypoproteinemia, malnutrition, malabsorption, nephrotic syndrome, ovarian failure, testosterone-producing tumors, protein-losing enteropathies, significant stress, and certain drugs including steroids, androgens, propranolol, phenytoin, and danazol (Pagana 2021).

High TBG is associated with phthalate exposure (Choi 2020), hypothyroidism (Azad 2011), pregnancy, oral contraceptives, hormone replacement therapy, estrogen-producing tumors, porphyria, infectious hepatitis, and certain drugs including estrogen, tamoxifen, and methadone (Pagana 2021).

Overview

Thyroid binding globulin (TBG) is the primary carrier of thyroid hormones in circulation. It delivers hormones to the cells and stabilizes their levels in the blood (Chakravarthy 2022). When TBG is increased, total T3 and T4 will also appear increased; when TBG is decreased, total T3 and T4 will appear decreased. Measuring TBG along with T3 and T4 provides insight into the causes of altered hormone levels and can help distinguish true hypo- or hyperthyroidism from binding protein changes (Pagana 2021). Levels of TBG can be increased with hypothyroidism and decreased with hyperthyroidism, but a full thyroid panel is required for clinical assessment (Azad 2011).

Endocrine-disrupting chemicals can interfere with thyroid function with associated changes in TBG. Data from 1,254 individuals participating in the Korean National Environmental Health Survey indicates that exposure to phthalates (plasticizing chemicals) was associated with increased TBG, peripheral deiodinase activity, and total T3, and decreased in total T4 and free T3 (Choi 2020).

References

Azad, Reza Mansourian. “Abnormal serum thyroid hormones concentration with healthy functional gland: a review on the metabolic role of thyroid hormones transporter proteins.” Pakistan journal of biological sciences : PJBS vol. 14,5 (2011): 313-26. doi:10.3923/pjbs.2011.313.326

Chakravarthy, Varshini. and Sehar Ejaz . “Thyroxine-Binding Globulin Deficiency.” StatPearls, StatPearls Publishing, 5 July 2022.

Choi, Sohyeon et al. “Thyroxine-binding globulin, peripheral deiodinase activity, and thyroid autoantibody status in association of phthalates and phenolic compounds with thyroid hormones in adult population.” Environment international vol. 140 (2020): 105783. doi:10.1016/j.envint.2020.105783

Dunlap, Dickson B. “Thyroid Function Tests.” Clinical Methods: The History, Physical, and Laboratory Examinations, edited by H Kenneth Walker et. al., 3rd ed., Butterworths, 1990.

Pagana, Kathleen Deska, et al. Mosby's Diagnostic and Laboratory Test Reference. 15th ed., Mosby, 2021.