Thyroxil Mechanism of Action

Pharmacologic Category: Belongs to the class of thyroid hormones. ATC code: H03AA01.
Pharmacology: Pharmacodynamics: Thyroxine (T4) is a naturally occurring hormone produced by the thyroid gland and converted to the more active hormone triiodothyronine (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 microgram thyroxine is equivalent in activity to 20 to 30 microgram liothyronin/triiodothyronine or 60 mg Thyroid BP and/or local pharmacopoeia specification.
Pharmacokinetics: Absorption: Following oral administration, the absorption of thyroxine is incomplete and variable, especially when taken with food. The amount absorbed increases during fasting conditions.
Distribution: Thyroxine is nearly totally bound to serum protein.
Metabolism: The main pathway for the metabolism of thyroxine (T4) is its conversion, by de-iodination, to the active metabolite triiodothyronine (T3). Further de-iodination of T4 and T3 leads to production of inactive products.
Elimination: Thyroxine 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 thyroxine is eliminated in the faeces and approximately 30 to 55% of a dose of thyroxine is excreted in the urine.