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Pharmacotherapeutic group: Thyroid hormone. ATC code: H03AA01.
Pharmacology: Pharmacodynamics: Levothyroxine sodium is the monosodium salt of the levorotary isomer of thyroxine.
Pharmacodynamic Effects: Thyroxine (T4) is a naturally occurring hormone produced by the thyroid gland and converted to the more active hormone tri-iodothyronine (T3) in peripheral tissues. The precise signals controlling the conversion of T4 to T3 within the cell are not known. The thyroid hormones are required for normal growth and development, particularly of the nervous system. They increase the resting or basal metabolic rate of the whole organism and have stimulatory effects on the heart, skeletal muscle, liver and kidney. Thyroid hormones enhance lipolysis and the utilization of carbohydrate.
100 micrograms levothyroxine is equivalent in activity to 20 to 30 micrograms liothyronine/tri-iodothyronine or 60 mg thyroid BP and/or local pharmacopoeia specification.
Pharmacokinetics: Absorption: Following oral administration, the absorption of levothyroxine is incomplete and variable, especially when taken with food. The amount absorbed increases during fasting conditions.
Distribution: Levothyroxine is nearly totally bound to serum protein.
Biotransformation: The main pathway for the metabolism of thyroxine (T4) is its conversion, by de-iodination, to the active metabolite tri-iodothyronine (T3). Further de-iodination of T4 and T3 leads to production of inactive products.
Elimination: Levothyroxine is eliminated slowly from the body with a half-life of approximately seven days in a normal person. This may be reduced in hyperthyroid states or increased in hypothyroid patients.
In man, approximately 20 to 40% of levothyroxine is eliminated in the faeces and approximately 30 to 55% of a dose of levothyroxine is excreted in the urine.
Special Patient Populations: Renal Impairment: Renal disease does not appear to have any significant effect on the disposition of levothyroxine.
Hepatic Impairment: Hepatic disease does not appear to have any significant effect on the disposition of levothyroxine.
Toxicology: Preclinical Safety Data: No additional data of relevance.
