Basics

Levothyroxine sodium is the sodium salt of the synthetically produced hormone L-thyroxine. The active ingredient is administered in this form as tablets.
Levothyroxin entspricht dem körpereigenen Schilddrüsenhormon Thyroxin (T4) und wird hauptsächlich zur Behandlung einer Schilddrüsenunterfunktion eingesetzt. Zudem kommt das Arzneimittel auch bei der Behandlung von Schilddrüsenkrebs zum Einsatz.

Due to their metabolism-activating and basal metabolic rate-increasing properties, thyroid hormones are sometimes abused as slimming agents.

Pharmacology

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.

Toxicity

Overdose of L-thyroxine results in symptoms typical of hyperthyroidism, such as weight loss, nervousness, insomnia, increased blood pressure, diarrhea, sweating, palpitations, cardiac arrhythmias, and fever.

Toxicological data

The oral LD50 value in rats is 20 mg/kg.

Author

Markus Falkenstätter, BSc

Pharmacy student

Markus Falkenstätter is a writer on pharmaceutical topics in Medikamio's medical editorial team. He is in the last semester of his pharmacy studies at the University of Vienna and loves scientific work in the field of natural sciences.

Reviewer

Mag. pharm. Stefanie Brandauer

Pharmacist

Stefanie Lehenauer has been a freelance writer for Medikamio since 2020 and studied pharmacy at the University of Vienna. She works as a pharmacist in Vienna and her passion is herbal medicines and their effects.

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