Thyroid hormone synthesis

T3 liothyronine is approximately four times as potent as T4 levothyroxine on a microgram for microgram basis.

Thyroid hormone synthesis

Thyroglobulin, the active groups of which are two molecules of the iodine-containing compound thyroxine, has a molecular weight ofThyroglobulin also contains thyroxine with two and three iodine atoms instead of four and tyrosine with one and two iodine atoms. Injection of… Anatomy of the thyroid gland The thyroid arises from a downward outpouching of the floor of the pharynxand a persisting remnant of this migration is known as a thyroglossal duct.

The gland itself consists of two oblong lobes lying on either side of the trachea windpipe and connected by a narrow band of tissue called the isthmus.

In normal adults the thyroid gland weighs 10 to 15 grams 0. The lobes of the gland, as well as the isthmus, contain many small globular sacs called follicles. The follicles are lined with follicular cells and are filled with a fluid known as colloid that contains the prohormone thyroglobulin.

The follicular cells contain the enzymes needed to synthesize thyroglobulin, as well as the enzymes needed to release thyroid hormone from thyroglobulin.

When thyroid hormones are needed, thyroglobulin is reabsorbed from the colloid in the follicular lumen into the cells, where it is split into its component parts, including the two thyroid hormones thyroxine T4 and triiodothyronine T3.

The hormones are then released, passing from the cells into the circulation. Biochemistry of thyroid hormone Thyroxine and triiodothyronine contain iodine and are formed from thyronines, which are composed of two molecules of the amino acid tyrosine.

Both iodine and tyrosine are acquired in the diet. Thyroxine contains four iodine atoms, and triiodothyronine contains three iodine atoms. Because each molecule of tyrosine binds one or two iodine atoms, two tyrosines are used to synthesize both thyroxine and triiodothyronine.

These two hormones are the only biologically active substances that contain iodine, and they cannot be produced in the absence of iodine. The process leading to the eventual synthesis of thyroxine and triiodothyronine begins in the thyroid follicular cells, which concentrate iodine from the serum.

The iodine is then oxidized and attached to tyrosine residues forming compounds called iodotyrosines within thyroglobulin molecules. The iodinated tyrosine residues are then rearranged to form thyroxine and triiodothyronine.

Therefore, thyroglobulin serves not only as the structure within which thyroxine and triiodothyronine are synthesized but also as the storage form of the two hormones. Considerably more thyroxine is produced and secreted by the thyroid gland than is triiodothyronine.

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However, thyroxine is converted to triiodothyronine in many tissues by the action of enzymes called deiodinases. After thyroxine enters a cell, deiodinases located in the cytoplasm remove one of its four iodine atoms, converting it into triiodothyronine. The triiodothyronine either enters the nucleus of the cell or is returned to the circulation.

As a result, all of the thyroxine and about 20 percent of the triiodothyronine produced each day come from the thyroid gland. The remaining 80 percent of triiodothyronine comes from deiodination of thyroxine outside of the thyroid. Most if not all of the action of thyroid hormone in its target tissues is exerted by triiodothyronine.

Therefore, thyroxine may be considered a circulating precursor of triiodothyronine. In serum more than 99 percent of the thyroxine and triiodothyronine is bound to one of three proteins.

Thyroid hormone synthesis

These binding proteins are known as thyroxine-binding globulin, transthyretin thyroxine-binding prealbuminand albumin. The remaining thyroxine and triiodothyronine less than 1 percent is free, or unbound.

When free hormone enters a cell, it is replenished immediately by hormone attached to the binding proteins. The binding proteins serve as reservoirs of the two hormones to protect the tissues from sudden surges of thyroid hormone production and probably also to facilitate delivery of the hormones to the cells of large, solid organs such as the liver.

Essentially all cells in the body are target cells of triiodothyronine.


Once triiodothyronine is inside a cell, it enters the nucleus, where it binds to proteins known as nuclear receptors. The triiodothyronine-receptor complexes then bind to deoxyribonucleic acid DNA molecules. This results in an increase in the rate at which the affected DNA molecules are transcribed to produce messenger ribonucleic acid mRNA molecules and an increase in the rate of synthesis of the protein translation coded for by the DNA by way of the mRNA.

Triiodothyronine increases the transcription of DNA molecules that code for many different proteins; however, it also inhibits the transcription of DNA that codes for certain other proteins. The patterns of activation and inhibition differ in different tissue and cell types. Actions of thyroid hormone The substances produced in increased quantities in response to triiodothyronine secretion include many enzymes, cell constituentsand hormones.

Key among them are proteins that regulate the utilization of nutrients and the consumption of oxygen by the mitochondria of cells. Mitochondria are the sites at which energy is produced in the form of adenosine triphosphate ATP or is dissipated in the form of heat.What Does the Thyroid Do?

The thyroid system affects every bodily function by regulating energy and heat production, growth, and tissue repair and development; stimulating protein synthesis; modulating carbohydrates, protein, fat metabolism, and digestion; modulating muscle and nerve action; and helping regulate hormone excretion and oxygen utilization.

Thyroid hormone synthesis is dependent on the cell polarity that conditions the targeting of specific membrane protein, either on the external side of the follicle (facing the blood capillaries) or on the internal side (at the cell-lumen boundary) and on the. The thyroid gland, or simply the thyroid, is an endocrine gland in the neck, consisting of two lobes connected by an is found at the front of the neck, below the Adam's thyroid gland secretes thyroid hormones, which primarily influence the metabolic rate and protein hormones also have many other effects including those on development.

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Receptors for thyroid hormones are intracellular DNA-binding proteins that function as hormone-responsive transcription factors, very similar conceptually to the receptors for steroid hormones.

Thyroid hormones enter cells through membrane transporter proteins. A number of plasma membrane. Table of Contents Checking the Thyroid – Self Test1 Testosterone Levels and Hypothyroidism Science behind Hypothyroidism Effect on Testosterone Lab Tests Treatment for Low Testosterone and Hypothyroidism Complications if Hypothyroidism is Left Untreated References: Testosterone is a hormone produced by ovaries in women and testicles in men.

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