USC ENDOCRINE LABORATORIES

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TEST INDEX


Thyroglobulin (Tg) Tg Antibody TSH Free T4 Total T4 Free T3 Total T3 TPO Antibody
THBR FT4I or FT3I TBG Calcitonin PTH


THYROXINE-BINDING GLOBULIN (TBG)

Methodology  Chemiluminescent Immunoassay

Performed  Mon - Fri

Reported  Mon - Fri

Collect  One 6mL RT (Min: 4mL RT)

Transport  1 mL refrigerated serum (Min: 0.5 mL)

Stability  Stored at 2 - 8°C for 2 days or 1 month at - 20°C

Unacceptable  The best qualitative result is obtained if the use of grossly hemolyzed or lipemic samples is avoided.

Reference Range  14 - 31µg/mL

CPT Code  84442

Information: Thryoxine-binding globulin (TBG), synthesized by the liver, is the major transport protein for the thyroid hormones, thyroxine, and triiodothyronine in the circulation, due to its characteristic of having the highest affinity for thyroid hormones compared to other thyroid hormone carriers, thyroxine-binding prealbumin, and albumin.  More than 99% of thyroid hormones are reversibly complexed to transport proteins and minute concentrations are free.  The body maintains the equilibrium between bound and unbound (free) T4.  This equilibrium will reassert itself despite alternation in the total level of T4 due to changes in TBG concentration and leave the free T4 level relatively unchanged.  For this reason, the free T4 concentration is more likely to correlate to the clinical thyroid status than the total T4 concentration.

Direct measurement of TBG is important to calculate T4/TBG ratio which correlates significantly with FT4 concentrations.  The ratio can be used as a substitute for FT4I when saturation of TBG binding sites by T4 is impaired (FT4I may give misleading results).

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