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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]


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.

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