Thyroid Function Testing

Thyroid function tests are performed by essentially all medical practitioners across every clinical setting. Significant insights have been gained into thyroid regulation and thyroid hormone metabolism that influence the selection and interpretation of thyroid function tests. Recent studies are summarized showing that genetic variations in key thyroid regulating genes may influence thyroid function tests. The basics of thyroid function measurements, approaches, limitations and clinical applications will be described for the major categories of thyroid function tests. Application of thyroid function testing to key clinical settings are discussed, including management of thyroid diseases. An approach to thyroid function testing in special groups is described, including infants and children, the elderly, pregnant women, and the hospitalized patient. The authors represent leaders in research and clinical practice from world class centers of excellence in medicine. "In this volume, leading experts provide a comprehensive overview of a difficult and commonly encountered clinical challenge--how to interpret thyroid function test results. Ranging from fundamental genetics to both inpatient and outpatient environments, this book will be of enriching value to internists, pediatricians, endocrinologists and those studying thyroid physiology." - Shlomo Melmed, M.D. Series Editor Thyroid function tests are utilized by essentially all medical practitioners, across every clinical setting, in patients from newborns to the elderly. They are the most frequently measured endocrine tests. The sensitive thyrotropin (TSH) assay reflects thyroid hormone feedback to the pituitary, and is diagnostic of both thyroid h- mone excess as well as deficiency. The log–linear relationship between serum TSH and thyroxine concentrations means that small changes in serum thyroxine are amplified by changes in serum TSH. The availability of the sensitive TSH assay in essentially all clinical laboratories has improved and simplified the assessment of thyroid function for the diagnosis of thyroid disease and to monitor treatment. Serum free thyroxine and thyrotropin concentrations, as well as other thyroid tests, can be measured utilizing an automated immunoassay platform that provides rapid and accurate results. This simplified approach to thyroid assessment, often requ- ing only a serum TSH measurement, and rapid availability of the thyroid function tests results, has expanded the scope of thyroid testing and clinicians ordering and interpretingth yroid tests. There remain, however, many challenges in selecting the appropriate thyroid function test to order, the correct interpretation of results, and applying these results to the diagnosis and management of thyroid diseases. It is especially important to be aware of limitations of thyroid function tests, as well as special clinical c- cumstances that can influence thyroid function measurements. The serum TSH concentration, for example, may not accurately reflect thyroid status in many si- ations including after prolonged hyperthyroidism when serum TSH remains s- pressed for months, in the presence of hypothalamic orpituitary disease, or due to a number of interfering medications. The serum free thyroxine, measured by the analog method, is not accurate with high or low serum binding proteins and d- ing pregnancy. Hospitalized patients often have thyroid function test abnormalities that are transient and return to normal after recovery from the acute illness. Iodine excessand deficiency candramatically influence thyroid function tests. Significant insights have been gained into the regulation of thyroid hormone synthesis and especially the role of thyroid hormone metabolism in supplying t- sues locally with an adequate supply of thyroid hormone. In a number of instances, these factors influence the selection and interpretation of thyroid function tests. Polymorphisms, common sequence variations, in genes of components that regulate thyroid function and thyroid hormone action may also contribute to variability in thyroid function tests in a population. v vi Preface This volume draws on an outstanding international panel of experts in thyroid function tests and thyroid function assessment. They represent clinicians, clinical researchers, and basic science researchers, all with a focus on some aspect of the assessment of thyroid function. The chapters all provide a clinical perspective, but are informed by themost recent scientific advancements. The first section of the book (Chaps. 1–3) presents the most recent advances in thyroid physiology, a review of genetic influences on thyroid function tests, and a discussion on the influence of iodine on thyroid function. In Chap. 1, Drs. Huang and de Castro Neves describe thyroid hormone metabolism, emphasizing the key role of thyroid hormone activation and inactivation in thyroid hormone action. Dr. Visser is a world leader in studies ofthyroid metabolism and genetic influences on thyroid function. In Chap. 2, Dr. Visser and his colleagues, Drs. van der Deure, Medici, and Peeters, provide a clear view of this important and r- idly expanding field. The population variation in the TSH “set point” (relationship between serum TSH and thyroxine in an individual), for example, is thought to be genetically determined, and influences the evaluati Thyroid function tests are utilized by essentially all medical practitioners, across every clinical setting, in patients from newborns to the elderly. They are the most frequently measured endocrine tests. The sensitive thyrotropin (TSH) assay reflects thyroid hormone feedback to the pituitary, and is diagnostic of both thyroid h- mone excess as well as deficiency. The log–linear relationship between serum TSH and thyroxine concentrations means that small changes in serum thyroxine are amplified by changes in serum TSH. The availability of the sensitive TSH assay in essentially all clinical laboratories has improved and simplified the assessment of thyroid function for the diagnosis of thyroid disease and to monitor treatment. Serum free thyroxine and thyrotropin concentrations, as well as other thyroid tests, can be measured utilizing an automated immunoassay platform that provides rapid and accurate results. This simplified approach to thyroid assessment, often requ- ing only a serum TSH measurement, and rapid availability of the thyroid function tests results, has expanded the scope of thyroid testing and clinicians ordering and interpretingth yroid tests. There remain, however, many challenges in selecting the appropriate thyroid function test to order, the correct interpretation of results, and applying these results to the diagnosis and management of thyroid diseases. It is especially important to be aware of limitations of thyroid function tests, as well as special clinical c- cumstances that can influence thyroid function measurements. The serum TSH concentration, for example, may not accurately reflect thyroid status in many si- ations including after prolonged hyperthyroidism when serum TSH remains s- pressed for months, in the presence of hypothalamic or pituitary disease, or due to a number of interfering medications. The serum free thyroxine, measured by the analog method, is not accurate with high or low serum binding proteins and d- ing pregnancy. Hospitalized patients often have thyroid function test abnormalities that are transient and return to normal after recovery from the acute illness. Iodine excessand deficiency candramatically influence thyroid function tests. Significant insights have been gained into the regulation of thyroid hormone synthesis and especially the role of thyroid hormone metabolism in supplying t- sues locally with an adequate supply of thyroid hormone. In a number of instances, these factors influence the selection and interpretation of thyroid function tests. Polymorphisms, common sequence variations, in genes of components that regulate thyroid function and thyroid hormone action may also contribute to variability in thyroid function tests in a population. v vi Preface This volume draws on an outstanding international panel of experts in thyroid function tests and thyroid function assessment. They represent clinicians, clinical researchers, and basic science researchers, all with a focus on some aspect of the assessment of thyroid function. The chapters all provide a clinical perspective, but are informed by themost recent scientific advancements. The first section of the book (Chaps. 1–3) presents the most recent advances in thyroid physiology, a review of genetic influences on thyroid function tests, and a discussion on the influence of iodine on thyroid function. In Chap. 1, Drs. Huang and de Castro Neves describe thyroid hormone metabolism, emphasizing the key role of thyroid hormone activation and inactivation in thyroid hormone action. Dr. Visser is a world leader in studies of thyroid metabolism and genetic influences on thyroid function. In Chap. 2, Dr. Visser and his colleagues, Drs. van der Deure, Medici, and Peeters,... Annotation Thyroid function tests are performed by essentially all medical practitioners across every clinical setting. Significant insights have been gained into thyroid regulation and thyroid hormone metabolism that influence the selection and interpretation of thyroid function tests. The first section of the book presents the recent advances in thyroid physiology and a discussion of genetic influences on thyroid function tests. The population variation in the TSH "set point", now thought to be genetically determined, influences the evaluation of thyroid function and thyroid function "targets" for treatment of thyroid disease. The basics of thyroid function measurements, approaches, limitations, and clinical applications will be described for the major categories of thyroid function tests. Application of thyroid function testing to the key clinical settings are discussed, including in the management of thyroid diseases. The influences of drugs on thyroid function testing remains a major clinical issue and is covered in a separate chapter and the issue of thyroid function test screening is also discussed. The authors represent leaders in research and thyroid clinical practice as well as representing a variety of outstanding institutions. This volume provides material that is clinically practical and informed by the most recent scientific advances Front Matter....Pages i-xix Thyroid Hormone Metabolism....Pages 1-20 Genetic Influences on Thyroid Function Tests....Pages 21-43 Influence of Iodine Deficiency and Excess on Thyroid Function Tests....Pages 45-69 Regulation of Thyroid Hormone Production and Measurement of Thyrotropin....Pages 71-84 Measurements of Thyroxine and Triiodothyronine....Pages 85-107 Thyroid Autoantibody Measurement....Pages 109-124 Thyroglobulin Measurement....Pages 125-153 Thyroid Function Testing in Ambulatory Practice....Pages 155-171 Assessing Thyroid Function in Infants and Children....Pages 173-185 Assessing Thyroid Function in Hospitalized Patients....Pages 187-207 Assessing Thyroid Function in Pregnancy....Pages 209-233 Assessing Thyroid Function in the Elderly....Pages 235-250 Influence of Drugs on Thyroid Function Tests....Pages 251-277 Back Matter....Pages 279-284 Thyroid function tests are performed by essentially all medical practitioners across every clinical setting. In this book, leading experts in the field provide clinically practical information from the most recent scientific advances.