Pharmacodynamics
Levothyroxine replaces the insufficiently produced hormone T4 in hypothyroidism, thereby increasing metabolic activity. It promotes the development and growth of tissues and bones and contributes to the maintenance of brain function and body temperature. Since endogenous thyroid hormone secretion is regulated by the hypothalamus, administration of high doses of levothyroxine inhibits thyroid-stimulating hormone secretion.
Pharmacokinetics
Orally administered levothyroxine is 40 to 80% absorbed. More than 99% of the thyroid hormones circulate plasma protein-bound. Most of the degradation process occurs in the liver, where the hormones are inactivated or broken down and subsequently excreted by the kidney.
Interactions
The rate of absorption can be reduced by foods that are increased at the same time, such as soybeans, milk, and dietary fiber. Drugs such as bile acid sequestrants, sucralfate, proton pump inhibitors and minerals (calcium, magnesium, iron, aluminum) can also negatively affect the absorption of the hormone. Therefore, it is recommended that levothyroxine be taken on an empty stomach at least 2 hours before a meal and at least 4 hours apart before taking agents that cause interactions.