Reference Values During Pregnancy

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Thyroxine-binding globulin (TBG) (serum)

   Units Nonpregnant Adult First Trimester Second Trimester Third Trimester


16 -24 10 - 40 23 - 46 19 - 49
μg/mL 16 -24 10 - 40 23 - 46 19 - 49
nmol/L 206 -309 171 - 684 393 - 786 325 - 838

Some causes of decreased TBG include nephrotic syndrome, cirrhosis of the liver, hereditary or idiopathic TBG deficiency, acromegaly, testosterone-producing tumors, severe acidosis, androgens, and glucocorticoids,

Some causes of increased TBG include estrogen (pregnancy, oral contraceptives, and estrogen-producing tumors),  acute liver disease, systemic illness ,acute intermittent porphyria, lymphocytic painless subacute thyroiditis,  hereditary or idiopathic TBG excess, and medications such as methadone, heroin, perphenazine, and clofibrate,

  • Thyroid binding globulin (TBG) levels rise during pregnancy in response to elevated estrogen levels.  Because the majority of T4 and T3 circulates bound to TBG the total T4 and total T3 measurements will also rise, but the levels of free T4 and free T3 will not be affected.


1. Lockitch G. Handbook of Diagnostic Biochemistry and Hematology in Normal Pregnancy. Boca Raton:CRC, 1993.
2.  Kratz A, Ferraro M, Sluss PM, Lewandrowski KB. Laboratory reference values. N Engl J Med. 2004;351(15):1548-15632. PMID:15470219
3. Brent GA.Maternal thyroid function: interpretation of thyroid function tests in pregnancy. Clin Obstet Gynecol. 1997 Mar;40(1):3-15.
PMID: 9103946
4. Fischbach FT, Dunning MB III, eds. A Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia, Lippincott Williams and Wilkins,2004

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