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triiodothyronine [T3] (free) in serum

Indications: - suspected T3 thyrotoxicosis * rarely necessary Reference interval: - 2.6-4.8 ng/L [1] (child & adult) - 3.6-5.6 ng/L [2] Clinical significance: - free T3 is the active form of plasma T3 - 0.2-0.5% of total plasma T3 exists unbound as free T3, - compare with free T4 0.02-0.04% - normally, T3 total in serum represents ~5% of thyroid hormone in serum - T3 is more active than T4, estimates range from 3-100 fold more active - serum free T3 is inversely related risk of mortality [6] - serum free T4 is positively related risk of mortality [6] * assays are highly variable [5] Method: - equilibirum dialysis immunoassay Specimen: - serum, refrigerate immediately, stable for 7 days at room temp, but storage at 4 C preferred

Related

T3 total in serum/plasma triiodothyronine (T3, liothyronine, Cytomel)

General

general chemistry test thyroid function test

References

  1. Clinical Guide to Laboratory Tests, 3rd edition, NW Tietz ed, WB Saunders, Philadelphia, 1995
  2. Medical Knowledge Self Assessment Program (MKSAP) 15, American College of Physicians, Philadelphia 2009
  3. Triiodothyronine, Free (Free T3) Laboratory Test Directory ARUP: 70133
  4. Free T3 by Equil Dialysis-TMS Laboratory Test Directory ARUP: 93243
  5. 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
  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