thyroxine

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thyroxine

(thīrŏk`sēn), substance secreted by the thyroid glandthyroid gland,
endocrine gland, situated in the neck, that secretes hormones necessary for growth and proper metabolism. It consists of two lobes connected by a narrow segment called the isthmus. The lobes lie on either side of the trachea, the isthmus in front of it.
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. The hormone thyroxine forms by combining the amino acid tyrosinetyrosine
, organic compound, one of the 20 amino acids commonly found in animal proteins. Only the l-stereoisomer appears in mammalian protein.
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 with iodine. Complexed to a protein, it is stored in the follicle stems between thyroid cells. Thyroxine enters into the bloodstream complexed to another protein, plasma globulin. Thyroxine increases the number and activity of mitochondria in cells by binding to the cells' DNA, increasing the basal metabolic rate. Administration of thyroid hormones, such as thyroxine, causes an increase in the rate of carbohydrate metabolism and a rise in the rate of protein synthesis and breakdown. The hormone, which excites the nervous system and leads to increased activity of the endocrine systemendocrine system
, body control system composed of a group of glands that maintain a stable internal environment by producing chemical regulatory substances called hormones.
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, remains active in the body for more than a month. Thyroxine activity is controlled by thyrotropinthyrotropin
or thyroid-stimulating hormone
(TSH), hormone released by the anterior pituitary gland that stimulates the thyroid gland to release thyroxine. The release of thyrotropin is triggered by the action of thyrotropin-releasing factor (TRF), a substance found in
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, a substance released from the pituitary glandpituitary gland,
small oval endocrine gland that lies at the base of the brain. It is sometimes called the master gland of the body because all the other endocrine glands depend on its secretions for stimulation (see endocrine system).
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. Conversely, thyroxine regulates the effect of thyrotropin by feedback inhibition, i.e., high levels of thyroxine depress the rate of thyrotropin secretion. Synthetically prepared thyroxine is used clinically in the treatment of thyroid gland deficiency diseases in adults and in the treatment of cretinismcretinism
, condition produced in infants and children due to lack of thyroid hormone. It usually results from a congenital defect (e.g., absence of the thyroid, presence of only a rudimentary gland, inability of the gland to produce thyroxine).
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 in children.

Thyroxine

A hormone secreted by the thyroid gland. Thyroxine (structure 1 ) is quite similar chemically and in biological activity to triiodothyronine ( 2 ).

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(1)
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.

Thyroxine

 

(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.

REFERENCES

Clegg, P., and A. Clegg. Gormony, kletki, organizm. Moscow, 1971. Chapter 9. (Translated from English.)
Tireoidnyegormony. Tashkent, 1972.
Kandror, V. I. “Nekotorye aktual’nye problemy mekhanizma dei-stviia tireoidnykh gormonov.” In Itogi nauki i tekhniki, vol. 11: Fiziologiia endokrinnoi sistemy. Moscow, 1973.
Hoch, F. L. “Biochemical Actions of Thyroid Hormones.” Physiological Reviews, 1962, vol. 42, no. 4.

I. V. KRIUKOVA

thyroxine

[thī′räk‚sēn]
(biochemistry)
C15H11I4NO4 The active physiologic principle of the thyroid gland; used in the form of the sodium salt for replacement therapy in states of hypothyroidism or absent thyroid function.