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Triiodothyronine
(S)-Triiodthyronine Structural Formulae.png
T3-3D-balls.png
IUPAC name
Other names triiodothyronine
T3
3,3',5-triiodo-L-thyronine
Identifiers
CAS number 6893-02-3 Yes check.svgY
SMILES
Properties
Molecular formula C15H12I3NO4
Molar mass 650.9776 g mol−1
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Triiodothyronine, C15H12I3NO4, also known as T3, is a thyroid hormone.

Thyroid-stimulating hormone (TSH) activates the production of thyroxine (T4) and T3. This process is under regulation. In the thyroid, T4 is converted to T3. TSH is inhibited mainly by T3. The thyroid gland releases greater amounts of T4 than T3, so plasma concentrations of T4 are 40-fold higher than those of T3. Most of the circulating T3 is formed peripherally by deiodination of T4 (85%), a process that involves the removal of iodine from carbon 5 on the outer ring of T4. Thus, T4 acts as prohormone for T3.

This thyroid hormone is similar to thyroxine but with one fewer iodine atoms per molecule. In addition, T3 exhibits greater activity and is produced in smaller quantity.

It is the most powerful thyroid hormone, and affects almost every process in the body, including body temperature, growth, and heart rate.

The biological halflife is 2.5 days.[1]

Contents

Production of T3

T3 is metabolically active hormone that is produced from T4. T4 is deiodinated by two deiodinases to produce the active triiodothyronine:
1. Type I present within the liver and accounts for 80% of the deiodination of T4
2. Type II present within the pituitary.

T4 is synthesised in the thyroid gland follicular cells as follows.
1. The Na+/I- symporter transports two sodium ions across the basement membrane of the follicular cells along with an iodine ion. This is a secondary active transporter that utilises the concentration gradient of Na+ to move I- against its concentration gradient.
2. I- is moved across the apical membranae into the colloid of the follicle.
3. Thyroperoxidase oxidises two I- to form I2. Iodide is non-reactive and only the more reactive iodine is required for the next step.
4. The thyroperoxidase iodinates the tyrosyl residues of the thyroglobulin within the colloid. The thyroglobulin was synthesised in the ER of the follicular cell and secreted into the colloid.
5. Thyroid stimulating hormone (TSH) released from the pituitary gland binds the TSH receptor ( a Gs protein coupled receptor) on the basolateral membrane of the cell and stimulates the endocytosis of the colloid.
6. The endosytosed vesicles fuse with the lysosomes of the follicular cell. The lysosomal enzymes cleave the T4 from the iodinated thyroglobulin.
7. These vesicles are then exocytosed releasing the thyroid hormones.

In the follicular lumen, tyrosine residues become iodinated. This reaction requires hydrogen peroxide. Iodine bonds carbon 3 or carbon 5 of tyrosine residues of thyroglobulin in a process called organification of iodine. The iodination of specific tyrosines yields monoiodotyrosine (MIT) and diiodotyrosine (DIT). One MIT and one DIT are enzymatically coupled to form T3. The enzyme is thyroid peroxidase.

Synthesis


Mechanism of Action

The T3 (and T4) bind to nuclear receptors, thyroid receptors. However, T3 (and T4) are not very lipophilic and as a result, are unable to pass through the phospholipid bilayers. They therefore have specific transport proteins on the cell membranes of the effector organs which allow the T3 and T4 to pass into the cells. The thyroid receptors bind to response elements in gene promoters and thus enabling them to activate or inhibit transcription. The sensitivity of a tissue to T3 is modulated through the thyroid receptors.

Transport of Triiodothyronine

The system of the thyroid hormones T3 and T4.[2]

T3 and T4 are carried in the blood bound to plasma proteins. This has the effect of increasing the half life of the hormone and decreasing the rate at which it is taken up by peripheral tissues. There are three main proteins that the two hormones are bound to. Thyronine binding globulin (TBG) is a gylcoprotein that has a higher affinity for T4 than for T3. The second plasma protein to which the hormone bind is transthyretin (which has a higher affinity for T3 than for T4). Both hormones bind with a low affinity to serum albumin, but due to the large availability of albumin it has a high capacity.

Effects of T3

T3 increases the basal metabolic rate and thus increases the body's oxygen and energy consumption. The basal metabolic rate is the minimal caloric requirement needed to sustain life in a resting individual. T3 acts on the majority of tissues within the body, with a few exceptions including the spleen and testis. It increases the production of the Na+/K+ -ATPase and in general increases the turnover of different endogenous macromolecules by increasing their synthesis and degradation.

Protein
T3 stimulates the production of RNA Polymerase I and II and therefore increases the rate of protein synthesis. It also increases the rate of protein degradation and in excess the rate of protein degradation exceeds the rate of protein synthesis. In such situations the body may go into negative ion balance.

Glucose
T3 potentiates the effects of the β-adrenergic receptors on the metabolism of glucose. It therefore increases the rate of glycogen breakdown and glucose synthesis in gluconeogenesis. It also potentiates the effects of insulin, which have opposing effects.

Lipids
T3 stimulates the breakdown of cholesterol and increases the number of LDL receptors, therefore increasing the rate of lipolysis.

T3 also affects the cardiovascular system. It increases the cardiac output by increasing the heart rate and force of contraction. This results in increased systolic blood pressure and decreased diastolic blood pressure. The latter two effects act to produce the typical bounding pulse seen in hyperthyroidism.

T3 also has profound effect upon the developing embryo and infants. It affects the lungs and influences the postnatal growth of the central nervous system. It stimulates the production of myelin, neurotransmitters and axon growth. It is also important in the linear growth of bones.

References

  1. ^ "Drug Information: Uses, Side Effects, Drug Interactions and Warnings" RxList
  2. ^ References used in image are found in image article in Commons:Commons:File:Thyroid_system.png#References.







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