Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1252 page)

   Limitations

   Various drugs can interfere with the test result.

THYROXINE-BINDING GLOBULIN (TBG)

   Definition

   This glycoprotein is the principal carrier for T
₃
and T
₄
. It declines with age in parallel with total and free T
₄
and T
₃
. The latter changes are accompanied by an increase in rT
₃
and rT
₃
index, suggesting a decrease in peripheral conversion of T
₄
and T
₃
rather than the change in secretory behavior of thyroid gland itself. With the availability of better free thyroid hormone tests, the TBG test is rarely used to assess the thyroid-binding hormone status.

   
Normal range:

   Male: 1.2–2.5 mg/dL

   Female: 1.4–3.0 mg/dL

   Use

   Diagnosis of genetic or idiopathic excess TBG

   Sometimes used to detect recurrent or metastatic differentiated thyroid carcinoma, especially follicular type and where the patient has had an increased level due to carcinoma

   To distinguish increased/decreased total T
₃
or total T
₄
concentrations due to changes in TBG; same purpose as T
₃
resin uptake and free thyroxine index

   Interpretation

Increased In

   Pregnancy.