Thyroid hormones play a central role in metbolic coordination. Robust control of thyroid homeostasis is therefore essential for survival and reproduction. In 1940, eleven years after the discovery of the thyrotropic hormone TSH by Aron in the year 1929, W. T. Salter postulated the existence of a feedback loop linking pituitary and thyroid gland, similarly to the two gonadotropic feedback control loops that had been described shortly before. Salter's hypothesis could be proved by Astwood and Hoskins only a few years later. In 1968, Panda and Turner succeeded in a quantitative description ot the interdependence of thyroxine and TSH levels, but the first mathematical models of the feedback loop had already been developed up to 12 years earlier. Over the years, cybernetic models of thyroid homeostasis were increasingly improved, a success that was also enabled by growing empirical data. As a consequence of this continued evolution, some of the newest models are also suitable for medical decision making. This book is a comprehensive description of one of these novel "parametrically isomorphic" models. It describes mathematical and computational aspects of this model as well as a diagnostic tool (SPINA Thyr) that allows for evaluating the thyroid's secretory capacity (GT) and sum activity of peripheral deiodinases (GD). Additionally, it describes reference ranges for GT and GD in a healthy population and the distributions of both structure parameters in patients suffering from selected thyroid diseases. A last section is devoted to diagnostic power of secretory capacity and deiodinase sum activity in terms of sensitivity, specificity and likelihood ratios.