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T3 total in serum/plasma

Indications: - suspected T3 thyrotoxicosis * rarely necessary [4] Contraindications: - not indicated in evaluation of subclinical hypothyroidism [5] Reference values: - 0.60 to 1.70 ng/mL. Clinical significance: - normally, T3 represents ~5% of thyroid hormone in serum - T3 has a greater intrinsic metabolic activity, faster turnover & larger volume of distribution than circulating T4 (levothyroxine) - thyrotoxicosis may be caused by abnormally high concentrations of T3 rather than T4 - serum T3 determination is important for monitoring hypothyroid patients receiving triiodothyronine (Cytomel) therapy - unlike 'T3 Uptake' tests, estimating saturation of thyroid hormone binding proteins, T3 analysis by RIA directly 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 - thus, T3 RIA values should not be used alone in establishing the thyroid status - levels of serum T4, serum TBG, serum TSH, & clinical findings needed ;;;********|**********|**********|**********|**********|*********|**********|********* Increases: - hyperthyroidism, T3 thyrotoxicosis Decreases: - hypothyroidism Principle: 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. Specimen: 1) Serum or heparinized plasma may be used. When serial samples are being evaluated, the same type of specimen should be used throughout the study. 2) 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. 3) Specimens showing particulate matter, erythrocytes, or turbidity should be centrifuged before testing. SAMPLE VOLUME: 200 uL of specimen is the minimum volume require to perform the assay.

Related

triiodothyronine (T3, liothyronine, Cytomel)

General

triiodothyronine in body fluid

References

  1. Diagnostic Products Corporation, 5700 West 96th Street, CA 90045, January 6, 1988.
  2. Henry, John Bernard., Evaluation of Endocrine Function, Clinical Diagnosis & Management, W.B. Saunder Co., Philadelphia, 1984, pp. 305-312
  3. Triiodothyronine, Total (Total T3) Laboratory Test Directory ARUP: 70474
  4. Krouss M et al. Free the T3: Implementation of best practice advisory to reduce unnecessary orders. Am J Med 2022 Dec; 135:1437. https://www.amjmed.com/article/S0002-9343(22)00571-X/fulltext
  5. Medical Knowledge Self Assessment Program (MKSAP) 19 American College of Physicians, Philadelphia 2022
  6. Lawton RI et al Longevity, demographic characteristics, and socio-economic status are linked to triiodothyronine levels in the general population. Proc Natl Acad Sci USA. 2014. Jan 9;121(2):e2308652121 PMID: 38175866 PMCID: PMC10786306 Free PMC article https://www.pnas.org/doi/10.1073/pnas.2308652121

Component-of

triiodothyronine (T3)/reverse triiodothyronine (T3) in serum/plasma triiodothyronine (T3)/thyroxine (T4) in serum/plasma