Thyroxine {2-amino-3- [4- (4-hydroxy-3,5-diiodophenoxy) -3,5-diiodophenyl] propionic acid is the main form of thyroid hormones in the thyroid gland . It is a prohormone to triiodothyronine .
Thyroxine is biologically inactive, in the peripheral tissues it is converted by the metalloenzyme of selenium-dependent monodeiodinase into the more active form - triiodothyronine . Thyroxine is formed by the addition of iodine to L-tyrosine .
From 2/3 to 4/5 of the total amount of thyroid hormones produced by the thyroid gland, enters the blood in the form of thyroxine, and only 1 / 3-1 / 5 - in the form of triiodothyronine.
The transport of thyroxine in the blood is carried out by the transthyretin proteins , thyroxin-binding globulin , albumin .
Content
Chemical Properties
The thyroxine molecule contains 4 iodine atoms. Most of the circulating blood thyroxin binds to thyroxin-binding globulin , which has a half-life of approximately 8 days.
Thyroxine Production
Thyroxine is produced by the follicular cells of the thyroid gland under the control of thyroid stimulating hormone (TSH) . Thyroxine tends to accumulate in the tissue of the thyroid gland. This hormone has a longer effect than many other hormones, so maintaining its constant level is vital for the body. The mechanism of thyroxine release from the thyroid gland into the blood is regulated by its concentration in the blood. Excess thyroxin suppresses its own secretion, inhibiting the release of thyroliberin (TRH) by the hypothalamus and thyroid stimulating hormone (TSH) hormone by the adenohypophysis. With a decrease in the level of thyroxin in the blood, its inhibitory effect on the secretion of TRH and TSH is removed. Prolonged cooling of the body, affecting the hypothalamic thermoregulation center, leads to the production of thyroliberin in the hypothalamus, thyroliberin acts on the adenohypophysis, which produces thyroid stimulating hormone (TSH), and this hormone acts on the thyroid gland, in which the synthesis and secretion of thyroxine is enhanced.
A small portion of thyroxine circulates in the blood in free form. In most cases, thyroxine transport is carried out in a bound form.
Functions
Thyroxine affects all body tissues; there are no specific target cells for it. This hormone is able to penetrate through the membrane and connect with receptors in every cell of the body.
The main function of thyroxine is the activation of metabolic processes, which is carried out through stimulation of the synthesis of RNA and corresponding proteins. Thyroxine affects the metabolism, increases body temperature, controls the growth and development of the body, increases protein synthesis and sensitivity to catecholamines, increases the heart rate, thickens the uterine mucosa. Enhances oxidative processes in the cells of the whole body, in particular brain cells. Thyroxine is important for the proper development and differentiation of all cells of the human body, and can also stimulate the metabolism of vitamins.
Discovery History
Edward Calvin Kendall first isolated thyroxine from dried thyroid preparations in 1919. Charles Robert Harington characterized and synthesized the hormone in 1926. In the same year, Georg Friedrich Henning released a thyroxine drug called Thyroxin Henning for those suffering from thyroid diseases.
Pathology
Excessive and inadequate thyroid activity may be accompanied by an increase in its size. With increased secretion of thyroxine, hyperthyroidism develops. An extreme degree of hyperthyroidism is called bazedovoy disease and can lead to heart failure . Lack of hormone or hypothyroidism at an early age can lead to cretinism , and at a more advanced age to myxedema .
Medical Use
L-thyroxine or levothyroxine is a synthetic analogue of thyroxine. It is used to normalize the thyroid gland with hypothyroidism . L-thyroxine stimulates the growth and development of tissues, increases their need for oxygen, stimulates metabolism, increases the functioning of the nervous and cardiovascular systems. At higher doses, L-thyroxine inhibits the production of hormones of the hypothalamus (TRH) and adenohypophysis (TSH). L-thyroxine is readily absorbed by the digestive system.
Notes
- β Walter F., PhD. Boron Medical Physiology: A Cellular And Molecular Approaoch. - Elsevier / Saunders, 2003. - P. 1300. - ISBN 1-4160-2328-3 .
Literature
- Chemical Encyclopedia / Editorial Board: Knunyants I.L. et al. - M .: Soviet Encyclopedia, 1995. - T. 4 (Half-Three). - 639 p. - ISBN 5-82270-092-4 .
- The norm and pathology of the human body / Parnes E.Ya .. - "Forum", 2012. - 285 p. - ISBN 978-5-91134-581-5 .
- Biology (in 3 volumes) / Taylor D., Green N., Stout W., edited by Soper R. .. - Moscow "MIR", 2001. - 435 p. - ISBN 5-03-003686-9 .
- Mashkovsky M.D. Medicines - "Medicine", Moscow, 1993. - 731 p. - ISBN 5-225-02734-2 .