Chemical formula: C₁₅H₁₁I₄NO₄ Molecular mass: 776.87 g/mol PubChem compound: 5819
The chief action of levothyroxine is to increase the rate of cell metabolism.
Levothyroxine sodium is used for the treatment of hypothyroidism. The thyroid gland is dependent upon 2 active principles for it’s main hormone activity these are Levothyroxine (tetraiodothyronine) and Tri-iodothyronine (see Goodman and Gilman, 1985). These closely related iodine containing amino acids are incorporated into the glycoprotein thyroglobulin. The chief action of Levothyroxine is to increase the rate of cell metabolism. Levothyroxine is deiodinated in peripheral tissues to form triiodothyronine which is thought to be the active tissue form of thyroid hormone. Triiodothyronine has a rapid action but a shorter duration of activity than levothyroxine.
Thyroxine (T4) is a naturally occurring hormone containing iodine, produced by the thyroid gland. It is converted to its more active principle triiodothyronine (T3) in the peripheral tissues. Receptors for T3 are found on cell membranes, mitochondria and cell nuclei. Thyroid hormones are required for normal growth and development of the body, especially the nervous system. They increase the basal metabolic rate of the whole body and have stimulatory effects on the heart, skeletal muscle, liver and kidney.
Levothyroxine sodium is incompletely and variably absorbed from the gastrointestinal tract. It is almost completely bound to plasma proteins and has a half-life in the circulation of about a week in healthy subjects, but longer in patients with myxoedema.
A large portion of the levothyroxine leaving the circulation is taken up by the liver. Part of a dose of levothyroxine is metabolised to triiodothyronine. Levothyroxine is excreted in the urine as free drug, deiodinated metabolites and conjugates. Some Levothyroxine is excreted in the faeces. There is limited placental transfer of levothyroxine.
Not applicable since levothyroxine has been used in clinical practice for many years and its effects in man are well known.
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