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thyroxine [T4] (free) in serum

Reference interval: - 0.8-2.3 ng/dL (adult) - 10.3-29.7 pmol/L Clinical significance: 1) free T4 is the active form of T4 2) 0.02-0.04% of total plasma T4 exists unbound as free T4, the majority is bound to 1 of 3 plasma proteins, albumin, thyroxine-binding globulin, prealbumin compare with free T3 0.2-0.5% 3) serum free thyroxine in the high-normal range associated with a 2.5 fold increased risk of sudden cardiac death [4] - absolute 10-year risk increases from 1% to 4% [4] 4) higher free T4 levels associated with increased risk for solid cancers (RR=1.8 for lung cancer, highest vs lowest tertile, lower risks for other solid cancers) [5] 5) serum free T4 is positively related risk of mortality [7] - serum free T3 is inversely related risk of mortality [7] Increases: - hyperthyroidism - high doses of biotin may falsely increase serum free thyroxine [6] Decreases: - hypothyroidism Method: - equilibirum dialysis immunoassay Specimen: - serum, refrigerate immediately, stable for 7 days at room temp, but storage at 4 C preferred

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thyroxine (T4)

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thyroxine [T4] (free) in body fluid thyroid function test

References

  1. Clinical Guide to Laboratory Tests, 3rd edition, NW Tietz ed, WB Saunders, Philadelphia, 1995
  2. Thyroxine Free Laboratory Test Directory ARUP: 70138
  3. Free T4 by Equil Dialysis-TMS Laboratory Test Directory ARUP: 93244
  4. Chaker L, van den Berg ME, Niemeijer MN et al Thyroid Function and Sudden Cardiac Death. A Prospective Population-Based Cohort Study. Circulation. September 6, 2016 PMID: 27601558 http://circ.ahajournals.org/content/134/10/713
  5. Khan SR, Chaker L, Ruiter R et al Thyroid Function and Cancer Risk: The Rotterdam Study. J Clin Endocrinol Metab. 2016 Sep 20:jc20162104. PMID: 27648963 http://press.endocrine.org/doi/abs/10.1210/jc.2016-2104
  6. Al-Salameh A et al A Somewhat Bizarre Case of Graves Disease Due to Vitamin Treatment. J Endo Soc. 2017;1(5):431-435. Not indexed in PubMed http://www.medscape.com/viewarticle/880576_2
  7. 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

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thyroid panel