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      Neonatal TSH Levels as an Index of Iodine Sufficiency: Differences Related to Time of Screening Sampling and Methodology

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          Abstract

          Introduction: Current WHO guidelines consider that under adequate iodine intake <3% of newborns should have neonatal TSH levels of >5 mU/l blood when screening is performed in cord blood or at 3 days to 3 weeks of age. Objective: To estimate whether this absolute criterion when applied to newborns older than 48 h of age and native to Buenos Aires coincides with the traditional ones (goiter and urinary iodine in school-age children (SAC)), and if the evaluation varies with either the methodology used for TSH measurements and/or the time of specimen sampling. Population and Methods: TSH was measured by an immunofluorometric assay (IFMA) on filter paper blood spots of 186 cord blood samples, 112 babies <48 h of age and 1,500 newborns >48 h of age, and by immunoradiometric assay (IRMA) in 238 newborns. The WHO ICCIDD absolute criteria were applied to each population. Thyroid volume was assessed by direct palpation in 500 SAC, and in 100 of them urinary iodine levels were measured. Results: TSH levels were >5 mU/l blood in 11.3% of the cord blood samples and in 3.6% of the samples from babies <48 h of age, suggesting mild iodine deficiency. TSH was >5 mU/l in 2.7% of newborns >48 h of age tested by IFMA (iodine sufficient) and in 30% measured by IRMA (moderate iodine insufficiency). Median urinary iodine and goiter prevalence in SAC were 143 µg/l and 4.5%, respectively, as expected in an iodine-sufficient area. Conclusion: The TSH levels in Buenos Aires conform with the WHO criterion that defines iodine sufficiency. Application of this criterion, however, to cord blood samples or samples from babies <48 h old and the use of different methodologies may lead to erroneous conclusions.

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          Most cited references 8

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          Micro determination of iodine by a catalytic method

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            Screening for congenital hypothyroidism used as an indicator of the degree of iodine deficiency and of its control.

             F Delange (1998)
            Neonatal thyroid screening using serum thyrotropin (TSH) as the primary screening test detects not only permanent sporadic congenital hypothyroidism, whose incidence is about 1 per 4000 births, but also compensated or transient primary hypothyroidism, whose incidence can be as high as 1 in 10 neonates and whose main cause is iodine deficiency. Elevated serum TSH in the neonate indicates insufficient supply of thyroid hormones to the developing brain, and therefore, constitutes the only indicator that allows prediction of possible impairment of mental development at a population level, which is the main consequence of iodine deficiency. Therefore, the World Health Organization (WHO), United Nations International Children's Emergency Fund (UNICEF), and the International Council for Control of Iodine Deficiency Disorders (ICCIDD) included neonatal TSH as one of the indicators for assessing iodine deficiency disorders (IDD) and their control. In the absence of iodine deficiency, the frequency of neonatal TSH above 5 mU/L whole blood (or 10 mU/L serum) is less than 3%. A frequency of 3%-19.9% indicates mild IDD. Frequencies of 20%-39.9% and above 40% indicate moderate and severe IDD, respectively. Neonates exhibit elevated serum TSH more frequently than adults for a similar degree of iodine deficiency. Consequently, they appear hypersensitive to the effects of iodine deficiency. This characteristic is explained by a particularly low iodine content of the thyroid of neonates and, consequently, by an accelerated turnover rate of their intrathyroidal iodine reserves. This turnover rate is 1% in adults. It is 17% in the neonate in conditions of iodine repletion, but is as high as 62% and 125% in conditions of moderate and severe iodine deficiency, respectively. Such an accelerated turnover rate requires thyroid hyperstimulation by TSH that is morphologically evidenced even in moderately iodine deficient neonates. Neonatal screening using primary TSH is implemented in most countries with mild IDD where it detects the cases of sporadic, permanent congenital hypothyroidism and where it is also used as a monitoring tool for IDD evaluation and control. However, the implementation of such programs in countries affected by moderate or severe IDD is still insufficient because of lack of resources of the countries. This should be considered in the framework of the external support often provided to these countries for the implementation of programs of universal salt iodization. Monitoring of these programs in order to achieve the goal of sustainable elimination of IDD now constitutes an absolute priority.
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              Two Simple Methods for Measuring Iodine in Urine

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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
                1663-2818
                1663-2826
                2004
                December 2004
                17 December 2004
                : 62
                : 6
                : 272-276
                Affiliations
                Fundación de Endocrinología Infantil, División de Endocrinología, Hospital de Niños R. Gutiérrez, Buenos Aires, Argentina
                Article
                81786 Horm Res 2004;62:272–276
                10.1159/000081786
                15523184
                © 2004 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
                Figures: 2, References: 26, Pages: 5
                Categories
                Original Paper

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