A multitude of physiological reactions and metabolic functions in the human body depend on the actions of thyroid hormones. For their proper functioning thyroid hormone concentrations must be kept in a narrow range both over long periods of time and during varying internal changes and imposed external conditions [1]. Tight control of circulating thyroid hormone concentrations is therefore a primary goal for attaining and maintaining a state of thyroid health [2]. This, in turn, creates a state of internal stability for the many biochemical reactions and metabolic processes that work at a narrow temperature optimum in the mammalian organism. Gaining control over energy expenditure independently of environmental short-term supply through the internal working of the thyroid system offers major systemic advantages [3]. This is a key role for the thyroid system which involves the hypothalamic–pituitary–thyroid axis regulation [1, 4]. A system failure may lead to thyroid dysfunction and hypothyroidism.