Triiodothyronine

Triiodothyronine, also known as T₃, is a thyroid hormone. It affects almost all physiological processes in the body, including growth and development, metabolism, body temperature and heart rate. Its function is to stimulate the metabolism of carbohydrates and fats, activating oxygen consumption, as well as the degradation of proteins within cells.

The production of T₃ and that of its prohormone thyroxine (T₄) is activated by thyrotropin (or TSH), which is secreted by the pituitary gland. in response to hypothalamic Thyrotropin Releasing Hormone (or TRH). This pathway is regulated through a closed-loop feedback process: elevated concentrations of T₃ and T₄ in blood plasma inhibit TSH production in the pituitary. and TRH in the hypothalamus. When the concentrations of these hormones decrease, the pituitary increases the production of TSH, and through these processes, a negative feedback control system is created to regulate the amount of thyroid hormones in the bloodstream.

The effects of T₃ on tissues are around four times more powerful than those of its prohormone T₄, since it binds with greater affinity to receptors. Of the thyroid hormones produced by the human body, only 20% is T₃, while 80% is T₄. Approximately 85% of circulating T₃ is formed through the elimination of the iodine atom attached to the number five carbon atom of T₄. The concentration of T₃ in human blood plasma is approximately one fortieth that of T₄. This is observed, in fact, due to the short half-life of T₃, which is only 2.5 days. In comparison, the half-life of T₄ sub> is 6.5 days.

Production of thyroid hormones

T₃ is a metabolically active hormone produced in the thyroid gland or by the deiodination of T₄. This is deiodinated by the enzymes iodothyronine deiodinases to produce triiodothyronine:

1. Type I present within the liver and represents 80% of T deyoding₄
   
3. Type II present within the pituitary gland.

Thyroid hormones are synthesized in the follicular cells of the thyroid gland in the following way:

2. Na's Secondary Active Carrier⁺/I^- transports two ions of sodium through the follicular cell membrane along with a iodine ion. The electrochemical gradient of the Na⁺ to move the I^- against yours.
   
4. The thyroid peroxidase oxidizes two I^- to form I₂The active form. The reducing power comes ultimately from glucose-6-phosphate
6. Simultaneously the thyroglobulin protein is synthesized in the endoplasmic reticulum of the follicular cell, it is processed in the golgi apparatus and is finally iodized by the same enzyme in position 3 (forming Monoyodothyrosine or MIT) or in positions 3 and 5 (forming Diyodothyrosine or DIT).
   
8. Once iodized it is transported to the outer zone of colloidal lumen where it dimerizes by action of the thyroid peroxidase with other MIT or DIT thylobulins, forming hormones triyodotironin and tetrayodothyronine or T4 plus other combinations that do not appear to be functional. They're all still attached to the thyroglobulin.
10. The thyrotropin released by the pituitary gland binds to its receptors in the basalt membrane of follicular cells (receptors coupled with G proteins) and stimulates the endocytosis of the coloid's vesicles through pseopodes.
    It also stimulates iodine capture.
12. Endocited vesicles containing iodine thyroglobulins are merged with lipsomas of follicular cells where proteasa activity separates from thyroglobulin both T3 and T4 thronins and MIT and DIT tyrosins.
    
14. T3 and T4 hormones are released into the blood by exocytosis in the apical membrane while thyroglobulin, MIT and DIT are denominated in the follicular and recycled cell.

Mechanism of action

The t ₃

(and t 4

) are fitted to nuclear receptors, thyroid hormone receptors. The t 3

(and t 4

) are quite lipophilic and are able to cross the phospholipid bilayers of the objective cells. The lipophilia of the t 3 (and t 4 ) require their fit to a conveyor protein called globulin thyroxine fixative (TBG) for blood transport. The sensitivity of the tissue to T 3 is modulated through the thyroid receptor.

TRIMODATE TRANSPORTATION

T₃ and T₄ are transported in the blood, attached 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. The two hormones attach to three main proteins:

• La globulin thyroxine fixer (TBG) is a glycoprotein that has a greater affinity for T₄ for the T₃.
• La transtiretina is also a glycoprotein, but with a greater affinity for T₃ for the T₄.

TriiodiRonin functions

Triiodyronine increases basal metabolism by increasing the use of oxygen and energy through the body. It acts in most tissues inside the body, with some exceptions including the spleen and testicles.

Effects on protein metabolism
The t ₃

stimulates the production of RNA polymerase I and II increasing the protein synthesis rate. This also increases the protein degradation rate, and, in excess, the protein degradation rate can exceed protein synthesis.

Effects on glucose metabolism
The T _{3 enhances the effects of adrenergic receptors on glucose metabolism. Therefore, the degradation rate of glycogen and gluconeogenesis increases. It also enhances the effect of insulin.}

Effects on lipid metabolism
The t ₃

stimulates cholesterol degradation and increases the number of LDL receptors by increasing the lipolysis rate. It also has effects on heart rate and contraction.

effects on heart rate

T 3 increases heart rate and contraction force, increasing levels of β-adrenergic receptors in myocardium. This results in an increase in systolic pressure and a decrease in a decrease in The diastolic pressure.

Development effects
The t ₃

has a deep effect on the development of the embryo as well as on children.

The t 3 affects the lungs and influence the postnatal growth of the central nervous system. Stimulates the production of myelin, neurotransmitters, and axons growth.

The t 3 is also important in the linear growth of the bones.

Effect on neurotransmitters
The t ₃

can increase serotonin levels in the brain, particularly in the cerebral cortex, and regulate down the receptors 5ht-2 (based on studies where T 3 reversed the helplessness learned in the physiological studies of the rats's brain).

In addition, T3 controls the biological rhythms mediated by the photoperiod. Their levels increase during spring-summer in long-day reproductive animals and decrease at the same time in short-day reproductive animals. This hormone influences the oxygenic and anorexigenic peptides system that control and direct the essential changes in the energy balance to support seasonal reproductive cycles.

Regulation of the secretion of T3

The thyroid hormone is stimulated by TSH (thyrotropin releasing hormone). On the other hand, the T3 inhibits the secretion of TSH in the pituitary and secondarily that of TRH in the paraventricular nucleus (PVN) of the hypothalamus.

T3 in the treatment of depressive disorders

The addition of triiodothyronine to existing treatments, such as ISRSS, is one of the most studied strategies for treatments resistant, however, success depends on the dose of T _{3 . A study without control and long -term Kelly and Lieberman of 17 patients with advanced unipolar depression resistant to treatments found an improvement in the symptoms of 14 patients in an average period of two years, in some cases with doses of T 3 higher than the traditional 50 mcg required to achieve therapeutic effects, with average dose of 80 mcg for 24 months (dose range: 25MCG-150mcg). The authors themselves published a retrospective study of 125 patients with three categories of bipolar disorder (i, II, us) whose treatment had been previously resistant to an average of 14 other treatments. They found that 84% experienced an improvement and 33% experienced total remission. None of the patients experienced hypomania while they were with T 3 .}

Use as a supplement for fat loss

3.5-Diiodo-l-gironine and 3.3 ' -diodo-l-gyronine are used as ingredients in certain supplements without a medical recipe for fat loss, designed for bodybuilding. Several studies have shown that these components increase the metabolization of fatty acids and burnt of adipose tissue in rats.

Alternative Medicine

Triiodyronine has been used to treat Wilson syndrome, an alternative medical diagnosis not recognized as a medical condition for current medicine. This diagnosis involves several non-specific symptoms that are attributed to the thyroid, despite having normal thyroid exams. The American thyroid association has aroused the concern that the treatment prescribed with triiodothyronine is potentially harmful.