|Values are valid only on day of printing.|
Normally, almost all thyroxine (99.5%) is bound to thyroxine-binding globulin, prealbumin, and albumin. Deficiencies and aberrant forms of these binding proteins can occur, causing difficulties interpreting thyroid function test results. Such abnormalities may be identified by thyroxine-binding protein electrophoresis.
Explaining unusual thyroxine (T4), free T4, and thyroxine-binding globulin (TBG) test results that do not correlate with the patientâ€™s clinical presentation.
Detecting the presence of aberrant thyroxine-binding proteins such as abnormal forms of albumin and prealbumin
Detecting selective deficiency of one of the thyroxine-binding proteins
Detecting antibodies to T4
An adjunct to the diagnosis of patients with high T4 concentration due to peripheral hormone resistance by ruling out thyroxine-binding abnormalities
Rare protein-binding abnormalities may be suspected in euthyroid patients having an elevated total thyroxine (T4) but normal thyroxine-binding globulin (TBG).
The following example is from a healthy 40-year-old male with familial dysalbuminemic hyperthyroxinemia, a benign familial condition that can be confused with hyperthyroidism:
-Increased T4 of 14.4 mcg/dL (normal=5.0-12.5 mcg/dL)
-Normal TBG of 20.1 mcg/dL (normal=12-26 mcg/mL)
The thyroxine-binding protein electrophoresis (TBPE) assay identified that of the saturating dose of (125)I-T4:
-52% was bound to albumin (normal 12%-34%)
-36% was bound to thyroxine-binding prealbumin (normal 49%-70%)
-13% was bound to TBG (normal 10%-25%)
In this example, based on the TBPE findings, this patientâ€™s increased serum T4 was determined due to increased binding to albumin. This was suggestive of familial dysalbuminemic hyperthyroxinemia (FDH), an inherited abnormality characterized by the presence of a variant serum albumin with preferential affinity for T4.
Thyroxine-binding globulin values may be elevated in females taking estrogens and during pregnancy.
This test should not be requested in patients who have recently received radioisotopes, therapeutically or diagnostically, because of potential assay interference. A recommended time period before collection cannot be made because it will depend on the isotope administered, the dose given, and the clearance rate in the individual patient.
THYROXINE-BINDING PROTEIN ELECTROPHORESIS
10.3-24.9 mcg T4/dL bound to TBG
11.5-34.1 mcg T4/dL bound to albumin
48.8-70.4 mcg T4/dL bound to prealbumin
1. Hay ID, Klee GG: Thyroid dysfunction. Endocrinol Metab Clin North Am 1988;17:473-509
2. Bartalena L, Robbins J: Thyroid hormone transport proteins. Clin Lab Med 1993;13(3):583-598