Diagnostic Thyroid Testing: Serum Total T4 and Total T3

Rodrigo Arrangoiz MS, MD, FACS, FSSO
  • Serum total T4 (TT4) and total T3 (TT3) concentrations:
    • Are a measure of both the bound and free hormone levels of these two hormones
  • TT4 or TT3 levels should be interpreted in the context of the clinical situation:
    • Because many clinical conditions and medications alter the concentrations of thyroid hormone binding proteins and / or compete with the binding of thyroid hormones to the binding proteins:
      • As such, measured TT4 and TT3 levels may be affected, even though the bioactive free levels and thus, the thyroidal status, remain unchanged
  • T3 is the active thyroid hormone:
    • It is primarily useful in the diagnosis and management of patients with hyperthyroidism
    • It occasionally can be used to differentiate stimulation induced thyrotoxicosis / Graves’ disease (TT3 / TT4 ratio > 20) from destruction induced thyrotoxicosis / subacute thyroiditis (TT3 / TT4 ratio < 12):
      • This assessment can be further augmented when TSH is considered as serum levels of TSH are generally suppressed in most untreated Graves’ patients, whereas they usually were not completely suppressed in patients with painless thyroiditis or subacute thyroiditis
    • Measurement of serum TT3 is not usually helpful if hypothyroidism is suspected:
      • Because the activity of 5’deiodinas type 2 enzyme (Dio2):
        • Which converts T4 to the biologically active T3:
          • Increases while serum T4 falls:
            • Thus maintaining normal T3 levels until the overall thyroid hormone levels are very low
  • rT3, which may be elevated during nonthyroidal illness:
    • Is not biologically active:
      • As such, the utility of measuring it and other forms of inactive iodothyronine are limited during the evaluation of thyroid status
  • Finally, the human anti-mouse antibodies (HAMAs) that interfere with TSH testing:
    • Can also interfere with the thyroid hormone assays
    • HAMA positivity:
      • May result in artificially elevated or reduced TT4, TT3, FT4, and FT3 levels
    • Patients who have received therapeutic monoclonal antibody treatment may be at increased risk of develop interfering positive HAMA titers
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Published by Rodrigo Arrangoiz MS, MD, FACS, FSSO

My name is Rodrigo Arrangoiz I am a breast surgeon/ thyroid surgeon / parathyroid surgeon / head and neck surgeon / surgical oncologist that works at Center for Advanced Surgical Oncology in Miami, Florida. I was trained as a surgeon at Michigan State University from (2005 to 2010) where I was a chief resident in 2010. My surgical oncology and head and neck training was performed at the Fox Chase Cancer Center in Philadelphia from 2010 to 2012. At the same time I underwent a masters in science (Clinical research for health professionals) at the University of Drexel. Through the International Federation of Head and Neck Societies / Memorial Sloan Kettering Cancer Center I performed a two year head and neck surgery and oncology / endocrine fellowship that ended in 2016. Mi nombre es Rodrigo Arrangoiz, soy cirujano oncólogo / cirujano de tumores de cabeza y cuello / cirujano endocrino que trabaja Center for Advanced Surgical Oncology en Miami, Florida. Fui entrenado como cirujano en Michigan State University (2005 a 2010 ) donde fui jefe de residentes en 2010. Mi formación en oncología quirúrgica y e n tumores de cabeza y cuello se realizó en el Fox Chase Cancer Center en Filadelfia de 2010 a 2012. Al mismo tiempo, me sometí a una maestría en ciencias (investigación clínica para profesionales de la salud) en la Universidad de Drexel. A través de la Federación Internacional de Sociedades de Cabeza y Cuello / Memorial Sloan Kettering Cancer Center realicé una sub especialidad en cirugía de cabeza y cuello / cirugia endocrina de dos años que terminó en 2016.

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