Thyroxine-binding globulin (TBG) is a protein that carries thyroid hormone through the blood and keeps levels stabilized. Measuring TBG along with T3 and T4 can help determine if changes in thyroid hormones are due to an increase or decrease in production or a change in TBG levels.
If TBG increases, then total T3 and T4 levels appear to increase as well, and if TBC decreases, then T3 and T4 appear to decrease. Although TBG can increase with hypothyroidism and decrease with hyperthyroidism, a full thyroid panel is recommended to truly distinguish thyroid dysfunction from alterations in binding proteins.
Decreased TBG can be seen with hyperthyroidism, malnutrition, hypoproteinemia, kidney dysfunction, major stress, and certain medications.
Increased TBG can be associated with hypothyroidism, pregnancy, oral contraceptives, estrogen-producing tumors, phthalate exposure, and certain medications.
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