Clinical Lab Handbook

Container/Tube: SST

Alternate Container: Red Top

Specimen Volume: 5 mL of serum

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

Chemiluminescence Immunoassay

A competitive immunoassay technique is used, which depends on a competition between T3 present in the sample with a horseradish peroxidase (HRP)-labeled T3 conjugate for a limited number of binding sites on a sheep anti‑T3 antibody presented in the liquid phase. Binding proteins effects are eliminated by use of an appropriate buffer and blocking agent. The antigen-antibody complex is captured by a donkey anti-sheep second antibody coated on the wells. Unbound materials are removed by washing.

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 T3 present.

Triiodothyronine is transported in serum primarily by thyroxine-binding globulin (TBG), approximately 99.5% of circulating T3 is protein-bound. T3 contributes significantly to the maintenance of the euthyroid state, and the total T3 concentration has a role in screening for thyroid disease in conjunction with other tests. T3 alone cannot diagnose hypothyroidism, but it may be more sensitive than thyroxine (T4) for hyperthyroidism. A fall in T3 concentrations of up to 50% is known to occur in a variety of clinical situations, including acute and chronic disease. T3 concentrations may be altered in conditions affecting the capacity of the thyroid hormone binding proteins, e.g. pregnancy when a test for T3 or T4 uptake may be used with the total T3 result to calculate the free T3 index (FT3I) to estimate the level of metabolically active free T3.

97-169 ng/dL

Monday through Sunday

84480