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 loglinear 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. 13) 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 evaluation of thyroid function and thyroid function targets for treatment of thyroid disease. Dr. Zimmerman, an int- nationally recognized expert in iodine, and his colleague, Dr. Andersson, provide in Chap. 3 an in-depth treatment of the most significant influence on thyroid function throughout the worldiodine intake. The influence of iodine deficiency and excess on individual thyroid function is discussed, as well as the population effects on t- roid diseases and especially fetal and neonatalde velop.