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      Measurement of Serum Free Thyroid Hormone Concentrations: An Essential Tool for the Diagnosis of Thyroid Dysfunction

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          Abstract

          Free thyroid hormones (free thyroxine, FT<sub>4</sub>, and free triiodothyronine, FT<sub>3</sub>) represent a more useful index of thyroid status than total thyroid hormones, because the latter are influenced by variations of thyroid hormone-binding proteins, especially T<sub>4</sub>-binding globulin (TBG). Thus, increased serum total T<sub>4</sub> (TT<sub>4</sub>) and, in many instances, T<sub>3</sub> (TT<sub>3</sub>) concentrations are encountered in euthyroid subjects with TBG excess, familial dysalbuminemic hyperthyroxinemia and transthyretin-associated hyperthyroxinemia, while decreased serum TT<sub>4</sub> and TT<sub>3</sub> levels are associated with TBG deficiency: under these circumstances, measurement of serum FT<sub>4</sub> and FT<sub>3</sub> levels correctly establishes the diagnosis of euthyroidism. In cases of suspected hyperthyroidism, a diagnostic strategy can be suggested based on serum FT<sub>3</sub> (and TSH) measurement, since FT<sub>4</sub> may occasionally be elevated, also in euthyroid subjects, e.g., in patients under chronic amiodarone or L-T<sub>4</sub> treatment. When hypothyroidism is suspected, the most reliable test appears to be FT<sub>4</sub> (together with TSH), because FT<sub>3</sub> may still be normal in patients with subclinical or mild thyroid failure. In any case, it is essential that reliable free thyroid hormone assays be used, which are devoid of methodological limitations responsible for artifactual results under particular circumstances, such as thyroid hormone-binding protein abnormalities, pregnancy and nonthyroidal illness.

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          Author and article information

          Journal
          HRE
          Horm Res Paediatr
          10.1159/issn.1663-2818
          Hormone Research in Paediatrics
          S. Karger AG
          978-3-8055-6290-4
          978-3-318-00047-4
          1663-2818
          1663-2826
          1996
          1996
          09 December 2008
          : 45
          : 3-5
          : 142-147
          Affiliations
          Istituto di Endocrinologia, University of Pisa, Italy
          Article
          184777 Horm Res 1996;45:142–147
          10.1159/000184777
          8964573
          © 1996 S. Karger AG, Basel

          Copyright: All rights reserved. No part of this publication may be translated into other languages, reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording, microcopying, or by any information storage and retrieval system, without permission in writing from the publisher. Drug Dosage: The authors and the publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accord with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any changes in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new and/or infrequently employed drug. Disclaimer: The statements, opinions and data contained in this publication are solely those of the individual authors and contributors and not of the publishers and the editor(s). The appearance of advertisements or/and product references in the publication is not a warranty, endorsement, or approval of the products or services advertised or of their effectiveness, quality or safety. The publisher and the editor(s) disclaim responsibility for any injury to persons or property resulting from any ideas, methods, instructions or products referred to in the content or advertisements.

          Page count
          Pages: 6
          Categories
          Hormone Binding Proteins: Physiology and Clinical Implications

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