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The thyroid gland has a powerful influence over the activities of the whole body. If the function of the gland is for any reason lost, serious disturbances of chemical and morphological structure arise and are manifested as physical and mental disorders. The diversity of the action of thyroid hormones on physiological functions, on the metabolic rate, and on the activity of various enzyme systems both in the body as a whole and in tissue preparations has been known for a long time. Our knowledge of the role of thyroid hormones is based on experiments in which a particular thyroid hormone has been given to healthy and athyrotic animals and on extensive clinical observations on patients with hyperthyroid and hypothyroid states.The effect of thyroid hormones in vivo is expressed as reactions of whole organs and of individual physiological functions and morphological structures. The internal secretions of the thyroid gland modify growth and metamorphosis, the utilization of oxygen by the body as a whole or by tissue preparations in vitro, various aspects of metabolism, and the activity of particular enzyme systems.The extreme diversity of the effects observed after injection of thyroid hormone may reflect the primary action of this agent on a single link fundamental to the mechanism of biochemical processes in the cell or on a single morphological structure responsible for the proper integration of processes at the cell level. These problems will be examined fully later. Here I shall merely state that when the action of a thyroid hormone is discussed, the term is used not only with respect to the thyroxine and triiodothyronine produced in the thyroid gland, but also to several of their derivatives and analogs. However, we know that all compounds with the biological activity 125 Y. K. Turakulov et al., Thyroid Hormones