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A hormone secreted by the thyroid gland. Thyroxine (structure 1 ) is quite similar chemically and in biological activity to triiodothyronine ( 2 ).Both are derivatives of the amino acid tyrosine and are unique in being the only iodine-containing compounds of importance in the economy of all higher forms of animal life. The thyroid gland avidly accumulates the small amount of iodine in the diet. This iodine is oxidized to iodide ion in the gland and then reacts with tryosine to form mono- and diiodotyrosine. These latter are then coupled to form either thyroxine or triiodothyronine. See Thyroid gland
The maintenance of a normal level of thyroxine is critically important for normal growth and development as well as for proper bodily function in the adult. Its absence leads to delayed or arrested development. It is one of the few hormones with general effects upon all tissues. Its lack leads to a decrease in the general metabolism of all cells, most characteristically measured as a decrease in nucleic acid and protein synthesis, and a slowing down of all major metabolic processes.
(3,5,3’,5’,-tetraiodothyronine), the principal thyroid hormone in vertebrate animals and man, produced by the thyroid follicles. Thyroxine is synthesized by iodination of the amino acid tyrosine and the oxidative condensation of two diiodotyrosine molecules with the splitting off of alanine. Like the other thyroid hormone, triiodothyronine, thyroxine is liberated during the enzymic breakdown of the complex it forms with the protein thyroglobulin. It combines with plasma proteins after entering the blood. Bound thyroxine is in a state of dynamic equilibrium with free thyroxine, which diffuses into the peripheral cells, where it performs its physiological functions.
In amphibians and some bony fishes, such as eels and flatfishes, thyroxine stimulates metamorphosis. In warm-blooded animals and man, it raises basal metabolism, thereby increasing heat production and affecting the growth and differentiation of tissues. The injection of thyroxine into the organism raises basal metabolism within 24 hours, increases oxygen consumption, accelerates the pulse rate, and increases the excitability of the nervous system.
High concentrations of thyroxine have been shown to disrupt oxidative phosphorylation in isolated mitochondria. Consequently, the energy of electron transfer in the respiratory chain is not stored in the form of high-energy compounds, such as ATP, but is released in the form of heat. The action of thyroxine on the enzymes of oxidative phosphorylation accounts for the hormone’s capacity to bind the metal ions needed for the activity of these enzymes.
The synthesis and secretion of thyroxine by the thyroid gland are regulated by thyrotropin, which is produced by the hypophysis. The production of thyroxine and thyrotropin is regulated by negative feedback. A rise in blood thyroxine levels inhibits the secretion of thyrotropin and thus decreases the secretion of thyroxine; a decrease in the concentration of thyroxine increases the secretion of thyrotropin, which restores the thyroxine balance. The secretion of thyroxine is also affected by environmental factors, such as temperature and stress, the presence of iodine in food, and the condition of the other endocrine glands. Disruption of the thyroxine and triiodothyronine balances in man gives rise to a variety of diseases.
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Hoch, F. L. “Biochemical Actions of Thyroid Hormones.” Physiological Reviews, 1962, vol. 42, no. 4.
I. V. KRIUKOVA