T3 Total In Serum

Reference-interval

The Coat-A-Count procedure is a solid-phase radioimmunoassay, wherein I-125 labeled T3 competes for a fixed time with T3 in the patient sample for sites on T3-specific antibody. This reaction takes place in the presence of blocking agents which serve to liberate bound triiodothyronine from carrier proteins; hence the assay measures total T3, since both free & protein-bound T3 from the patient sample are able to compete with radiolabeled T3 for antibody sites. The antibody being immobilized to the wall of a polypropylene tube decanting the supernatant suffices to terminate the competition & to isolate the antibody-bound fraction of the radiolabeled T3. Counting the tube in the gamma counter then yields a number, which converts by way of a calibration curve to a measure of the T3 present in the patient sample.

Under normal physiological conditions T3 represents approximately 5% of the thyroid hormones in serum. Although present in lower concentration, T3 has a greater intrinsic metabolic activity, faster turnover & larger volume of distribution than circulating T4. Thyrotoxicosis may be caused by abnormally high concentrations of T3-rather than T4-have reinforces the importance of T3 RIA measurements. T3 determination is also an important tool for monitoring hypothyroid patients receiving triiodothyronine ( Cytomel) therapy. Unlike ' T3 Uptake' tests, which estimate the saturation of thyroid hormone binding proteins, T3 analysis by RIA actually measures circulating levels of triiodothyronine. Most reports indicate that T3 levels distinguish clearly between euthyroid & hyperthyroid subjects, but provide a less clear-cut separation between hypothyroid & normal subjects.
Numerous conditions unrelated to thyroid disease may cause abnormal T3 values. Consequently, T3 RIA values should not be used on their own in establishing the thyroid status of an individual. The levels of serum T4, TBG, TSH, & other clinical findings must be considered as well.

Serum or heparinized plasma may be used. When serial samples are being evaluated, the same type of specimen should be used throughout the study.
If assay is performed within 24 hours after collection, the specimen should be stored in the refrigerator at 2-8*C. If the testing will be delayed more than 24 hours, the specimen should be frozen. Mix thoroughly after thawing to ensure consistency in the results. Avoid repeated freezing & thawing.
Specimens showing particulate matter, erythrocytes, or turbidity should be centrifuged before testing.

Diagnostic Products Corporation, 5700 West 96th Street, CA 90045, January 6, 1988.
Henry, John Bernard., Evaluation of Endocrine Function, Clinical Diagnosis & Management, W.B. Saunder Co., Philadelphia, 1984, pp. 305-312

T3 total in serum/plasma