|Values are valid only on day of printing.|
Normally, almost all (99.5%) thyroxine 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 clinical condition of a patient
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
As 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 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%)
No significant cautionary statements
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
Other protein binders: normally not present
Thyroxine-binding globulin values may be elevated in females taking estrogens and by pregnancy.
> or =12 months: 5.0-12.5 mcg/dL
Reference values have not been established for patients that are <12 months of age.
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