T3 total in serum/plasma
Indications
- suspected T3 thyrotoxicosis
* rarely necessary[4]
Contraindications
- not indicated in evaluation of subclinical hypothyroidism[5]
Reference interval
- 0.60 to 1.70 ng/mL.
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.
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
Decreases
Specimen
- 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.
SAMPLE VOLUME: 200 uL of specimen is the minimum volume require to perform the assay.
More general terms
Additional terms
Component of
- triiodothyronine (T3)/reverse triiodothyronine (T3) in serum/plasma
- triiodothyronine (T3)/thyroxine (T4) in serum/plasma
References
- ↑ 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
- ↑ Triiodothyronine, Total (Total T3) Laboratory Test Directory ARUP: http://www.aruplab.com/guides/ug/tests/0070474.jsp
- ↑ 4.0 4.1 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
- ↑ 5.0 5.1 Medical Knowledge Self Assessment Program (MKSAP) 19 American College of Physicians, Philadelphia 2022
- ↑ 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: https://www.ncbi.nlm.nih.gov/pubmed/38175866 PMCID: PMC10786306 Free PMC article https://www.pnas.org/doi/10.1073/pnas.2308652121