Clinical Lab Handbook

Container/Tube: SST

Alternate Specimen: Red Top

Specimen Volume: 5 mL of serum

Collection Instructions: Allow specimen to clot for 30 minutes. Spin specimen in centrifuge.

Chemiluminescence Immunoassay

A direct, labeled antibody, competitive immunoassay technique is used. FT3 present in the sample competes with ligand on the modified well surface for a limited number of binding sites on a horseradish peroxidase (HRP)-labeled antibody conjugate (sheep anti-T3). The well surface has been modified to act as a ligand for uncombined conjugate. Unbound material is removed by washing. The test design, with optimal reagent concentrations, ensures that disturbance of the T3/ binding protein equilibrium is so small as to be negligible.

The bound HRP conjugate is measured by a luminescent reaction. A reagent containing luminogenic substrates (a luminol derivative and a peracid salt) and an electron transfer agent, is added to the wells. The HRP in the bound conjugate catalyzes the oxidation of the luminol derivative, producing light. The electron transfer agent (a substituted acetanilide) increases the level of light produced and prolongs its emission. The light signals are read by the system. The amount of HRP conjugate bound is indirectly proportional to the concentration of FT3 present.

The free fraction of the circulating triiodothyronine (T3) is considered to exert the main influence on metabolic control. In hyperthyroidism, FT3 concentrations are generally elevated and give efficient discrimination at the euthyroid/toxic borderline, providing an effective method for confirming hyperthyroidism and monitoring of its treatment. In hypothyroidism, FT3 concentrations tend to be lower, but the decrease is insufficient to give clear diagnostic information. FT3 concentrations are independent of the concentration of thyroid hormone-binding proteins, and may be measured in patients with elevated or reduced binding protein concentrations without the need for additional tests of binding capacity. FT3 determinations should be used as part of a thyroid test strategy, which may include free T4 and high sensitivity TSH tests.

2.5-3.9 ng/mL

Monday through Sunday

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