Thyroxine-Binding Globulin (TBG)
Thyroxine-Binding Globulin (TBG) — Trimester-Specific Reference Ranges
TBG increases substantially in pregnancy due to estrogen stimulation of hepatic protein synthesis, resulting in higher total T4/T3 levels while free hormone concentrations remain normal.
| Units | Nonpregnant Adult | 1st Trimester | 2nd Trimester | 3rd Trimester |
|---|---|---|---|---|
| mg/L | 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 |
Physiologic changes in pregnancy
- Estrogen increases hepatic synthesis of TBG.
- Reduced clearance of sialylated TBG prolongs circulating levels.
- Total T4 and T3 rise proportionally with increased TBG.
- Free T4 and free T3 remain unchanged in healthy pregnancy.
Causes of low TBG
- Nephrotic syndrome
- Liver cirrhosis / hepatic failure
- Hereditary TBG deficiency
- Acromegaly
- Androgen therapy / testosterone excess
- Severe acidosis
- Glucocorticoid therapy
Causes of high TBG
- Estrogen excess (pregnancy, OCPs, estrogen-producing tumors)
- Acute liver disease
- Systemic illness
- Hereditary TBG excess
- Drugs: methadone, heroin, clofibrate, perphenazine
- Subacute/painless thyroiditis
References
- Lockitch G. Handbook of Diagnostic Biochemistry and Hematology in Normal Pregnancy. CRC Press; 1993.
- Kratz A, Ferraro M, Sluss PM, Lewandrowski KB. Laboratory reference values. NEJM. 2004;351:1548–1563.
- Brent GA. Maternal thyroid function: pregnancy interpretation. Clin Obstet Gynecol. 1997;40:3–15.
- Fischbach FT, Dunning MB. Manual of Laboratory and Diagnostic Tests, 7th ed. LWW; 2004.