Thyroid hormone

Thyroid hormone

The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are tyrosine-based hormones produced by the thyroid gland. An important component in the synthesis is iodine. The major form of thyroid hormone in the blood is thyroxine (T4). The ratio of T4 to T3 released in the blood is roughly 20 to 1. Thyroxine is converted to the active T3 (three to four times more potent than T4) within cells by deiodinases (5'-iodinase). These are further processed by decarboxylation and deiodination to produce iodothyronamine (T1a) and thyronamine (T0a).


Most of the thyroid hormone circulating in the blood is bound to transport proteins. Only a very small fraction of the circulating hormone is free (unbound) and biologically active, hence measuring concentrations of free thyroid hormones is of great diagnostic value.

When thyroid hormone is bound, it is not active, so the amount of free T3/T4 is what is important. For this reason, measuring total thyroxine in the blood can be misleading.

T3 and T4 cross the cell membrane, probably via amino acid importins, and function via a well-studied set of nuclear receptors in the nucleus of the cell, the thyroid hormone receptors.

T1a and T0a are positively charged and do not cross the membrane; they are believed to function via the trace amine-associated receptor Gene|TAAR1 (TAR1, TA1), a G-protein-coupled receptor located in the cell membrane.

Another critical diagnostic tool is measurement of the amount of thyroid-stimulating hormone (TSH) that is present.


The thyronines act on the body to increase the basal metabolic rate, affect protein synthesis and increase the body's sensitivity to catecholamines (such as adrenaline) by permissiveness. The thyroid hormones are essential to proper development and differentiation of all cells of the human body. These hormones also regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic compounds. They also stimulate vitamin metabolism. Numerous physiological and pathological stimuli influence thyroid hormone synthesis.

Thyroid hormone leads to heat generation in humans. However, the thyronamines function via some unknown mechanism to inhibit neuronal activity; this plays an important role in the hibernation cycles of mammals and the moulting behaviour of birds. One effect of administering the thyronamines is a severe drop in body temperature.

Related diseases

Both excess and deficiency of thyroxine can cause disorders.

* Thyrotoxicosis or hyperthyroidism (an example is Graves Disease) is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men.
* Hypothyroidism (an example is Hashimoto's thyroiditis) is the case where there is a deficiency of thyroxine, triiodiothyronine, or both.
* Clinical depression can sometimes be caused by hypothyroidismcite journal
author = Kirkegaard C, Faber J
title = The role of thyroid hormones in depression
journal = Eur J Endocrinol
volume = 138
issue = 1
pages = 1–9
year = 1998
pmid = 9461307
doi = 10.1530/eje.0.1380001
] . Some researchcite journal
author = Dratman M, Gordon J
title = Thyroid hormones as neurotransmitters
journal = Thyroid
volume = 6
issue = 6
pages = 639–47
year = 1996
pmid = 9001201
] has shown that T3 is found in the junctions of synapses, and regulates the amounts and activity of serotonin, norepinephrine, and Gamma-aminobutyric acid (GABA) in the brain.

Medical use of thyroid hormones

Both T3 and T4 are used to treat thyroid hormone deficiency (hypothyroidism). They are both absorbed well by the gut, so can be given orally. Levothyroxine, the most commonly used synthetic thyroxine form, is a stereoisomer of physiological thyroxine, which is metabolised more slowly and hence usually only needs once-daily administration. Natural desiccated thyroid hormones, also under the commercial name Armour Thyroid, is derived from pig thyroid glands, it is a "natural" hypothyroid treatment containing 20% T3 and traces of T2, T1 and calcitonin.Also available are synthetic combinations of T3/T4 in different ratios (such as Thyrolar) and pure-T3 medications (Cytomel).

Thyronamines have no medical usages yet, though their use has been proposed for controlled induction of hypothermia which causes the brain to enter a protective cycle, useful in preventing damage during ischemic shock.

Synthetic thyroxine was first successfully produced by Charles Robert Harington and George Barger in 1926.

Production of the thyroid hormones

Thyroxine (3,5,3',5'-tetra­iodothyronine) is produced by follicular cells of the thyroid gland. It is produced as the precursor thyroglobulin (this is "not" the same as TBG), which is cleaved by enzymes to produce active T4.

Thyroxine is produced by attaching iodine atoms to the ring structures of tyrosine molecules. Thyroxine (T4) contains four iodine atoms. Triiodothyronine (T3) is identical to T4, but it has one less iodine atom per molecule.

Iodide is actively absorbed from the bloodstream by a process called 'iodine trapping' and concentrated in the thyroid follicles. (If there is a deficiency of dietary iodine, the thyroid enlarges in an attempt to trap more iodine, resulting in goitre.) Via a reaction with the enzyme thyroperoxidase, iodine is covalently bound to tyrosine residues in the thyroglobulin molecules, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). Linking two moieties of DIT produces thyroxine. Combining one particle of MITand one particle of DIT produces triiodothyronine.

* DIT + MIT → r-T3 (biologically inactive)
* MIT + DIT → triiodothyronine (usually referred to as T3)
* DIT + DIT → thyroxine (referred to as T4)

Proteases digest iodinated thyroglobulin, releasing the hormones T4 and T3, the biologically active agents central to metabolic regulation. Thyroxine is supposedly a prohormone and a reservoir for the most active and main thyroid hormone T3. T4 is converted as required in the tissues by deiodinases. Deficiency of deiodinase can mimic an iodine deficiency. T3 is more active than T4 and is the final form of the hormone, though it is present in less quantity than T4.

Anti-thyroid drugs

Iodine uptake against a concentration gradient is mediated by a sodium iodine symporter. Perchlorate and thiocyanate are drugs that can compete with iodine at this point.

Effects of thyroxine

* Increases cardiac output
* Increases heart rate
* Increases ventilation rate
* Increases basal metabolic rate
* Potentiates the effects of catecholamines (i.e increases sympathetic activity)
* Potentiates brain development
* Thickens endometrium in females


See also

* Hormone
* Thyroid gland
* Thyroid-stimulating hormone
* Thyronamines, metabolites of the thyroid hormones that act at the trace amine-associated receptor TAAR1 (TAR1)
* Goitre
* Graves-Basedow disease

External links

* [ Collection of medical articles on Thyroid disease including the hormones]
* [ Find TH response elements in DNA sequences.]
* [ Collection of references to articles comparing different treatment methods of hypothyroidism]

Wikimedia Foundation. 2010.

Игры ⚽ Нужна курсовая?

Look at other dictionaries:

  • thyroid hormone — n any of several closely related metabolically active compounds (as triiodothyronine) that are stored in the thyroid gland in the form of thyroglobulin or circulate in the blood usu. bound to plasma proteins esp THYROXINE * * * an iodine… …   Medical dictionary

  • thyroid hormone — A hormone that affects heart rate, blood pressure, body temperature, and weight. Thyroid hormone is made by the thyroid gland and can also be made in the laboratory …   English dictionary of cancer terms

  • Thyroid hormone receptor beta — Thyroid hormone receptor, beta (erythroblastic leukemia viral (v erb a) oncogene homolog 2, avian) PDB rendering based on 1bsx …   Wikipedia

  • Thyroid hormone receptor alpha — Thyroid hormone receptor, alpha PDB rendering based on 1nav …   Wikipedia

  • Thyroid hormone resistance — MedlinePlus = eMedicineSubj = eMedicineTopic = MeshID = D018382 Thyroid hormone resistance describes a rare syndrome where the thyroid hormone levels are elevated but the thyroid stimulating hormone (TSH) level is not suppressed, or not… …   Wikipedia

  • Thyroid hormone receptor — protein Name=thyroid hormone receptor, alpha (erythroblastic leukemia viral (v erb a) oncogene homolog, avian) caption= width= HGNCid=11796 Symbol=THRA AltSymbols=THRA1, THRA2, ERBA1 EntrezGene=7067 OMIM=190120 RefSeq=NM 199334 UniProt=P10827 PDB …   Wikipedia

  • Thyroid hormone organification defect IIb — This condition, clinically called Pendred syndrome, is the hereditary association of congenital deafness (deafness at the time of birth) and goiter (enlargement of the thyroid gland in the front of the neck) due to a defect in the making of… …   Medical dictionary

  • Thyroid-hormone transaminase — In enzymology, a thyroid hormone transaminase (EC number| is an enzyme that catalyzes the chemical reaction:L 3,5,3 triiodothyronine + 2 oxoglutarate ightleftharpoons 3 [4 (4 hydroxy 3 iodophenoxy) 3,5 diiodophenyl] 2 oxopropanoate + L… …   Wikipedia

  • thyroid hormone treatment — Treatment with thyroid hormone, which is a hormone that affects heart rate, blood pressure, body temperature, and weight …   English dictionary of cancer terms

  • thyroid hormone — noun any of several closely related compounds that are produced by the thyroid gland and are active metabolically (Freq. 4) • Hypernyms: ↑hormone, ↑endocrine, ↑internal secretion • Hyponyms: ↑calcitonin, ↑thyrocalcitonin, ↑ …   Useful english dictionary

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”