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.