Biochemical testing is a cornerstone in the management of hypothyroidism. Laboratory thyroid tests have seen impressive technical advances over the past decades [1–3]. They are essential for the accurate diagnosis of thyroid disorders because signs and symptoms of the disease are frequently subtle in many patients with hypothyroidism, making biochemical testing necessary to detect and/or confirm the disease [4]. This view has been taken one step further, leading to the introduction of an entirely new category of disease, subclinical hypothyroidism, and its counterpart subclinical hyperthyroidism [5]. The definition of disease has thereby been separated from the clinical expression of any signs and symptoms of a thyroid dysfunction and is solely based on biochemical testing. It solely relies on the reliability of the measurement of TSH as the presumed single most sensitive thyroid parameter. In Chap. 5 we thoroughly examined and questioned this practice on the basis of an improved and extended understanding of hypothalamic–pituitary–thyroid regulation. This redefines the role of TSH in the thyroid system and, consequently, as a clinical utility. The focus has recently shifted back from an over-reliance on TSH to the need of direct measurement of thyroid hormones [6–9]. In Chap. 9 , we take a closer look at the quality of the methods used for thyroid function testing, choice of hormones to be tested and assays best suited for optimisation of the diagnostic process.