Euthyrox Mechanism of Action

Pharmacological class: Thyroid hormones. ATC code: H03A A01.
Pharmacology: Pharmacodynamics: The synthetic levothyroxine is identical in effect with the naturally occurring major hormone secreted by the thyroid. It is converted to T₃ in peripheral organs and, like the endogenous hormone, develops its specific effects at the T₃ receptors. The body is not able to differentiate between endogenous and exogenous levothyroxine.
Pharmacokinetics: Orally given levothyroxine is absorbed almost exclusively in the upper small intestine. Depending on the galenic formulation absorption amounts up to 80%. t_max is approximately 5 - 6 hours.
Following oral administration the onset of action is seen after 3 - 5 days. Levothyroxine exhibits an extremely high binding to specific transport proteins of about 99.97%. This protein hormone binding is not covalent and so the bound hormone in plasma is in continuous and very rapid exchange with the fraction of the free hormone.
Due to its high protein binding levothyroxine cannot be eliminated via haemodialysis or haemoperfusion.
The half-life of levothyroxine is on average 7 days. In hyperthyroidism it is shorter (3 -4 days) and in hypothyroidism it is longer (approx. 9 - 10 days). The volume of distribution amounts to about 10 - 12 l. The liver contains ¹/₃ of the entire extra-thyroidal levothyroxine, which is rapidly exchangeable with the levothyroxine in serum. Thyroid hormones are metabolised mainly in the liver, kidneys, brain and muscles. The metabolites are excreted with urine and faeces. The overall metabolic clearance for levothyroxine is about 1.2 l plasma per day.