ATC Code V04CJ03
Formula C15H11I4NO4
Molar Mass (g·mol−1) 776.874
Physical State solid
Melting Point (°C) 231 to 233
PKS Value 0.27
CAS Number 51-48-9
PUB Number 5819
Drugbank ID DB00451
Solubility soluble in water


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.



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.


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.


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.


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.

Markus Falkenstätter

Markus Falkenstätter

Markus Falkenstätter ist Autor zu pharmazeutischen Themen in der Medizin-Redaktion von Medikamio. Er befindet sich im letzten Semester seines Pharmaziestudiums an der Universität Wien und liebt das wissenschaftliche Arbeiten im Bereich der Naturwissenschaften.

Mag. pharm Stefanie Lehenauer

Mag. pharm Stefanie Lehenauer

Stefanie Lehenauer ist seit 2020 freie Autorin bei Medikamio und studierte Pharmazie an der Universität Wien. Sie arbeitet als Apothekerin in Wien und ihre Leidenschaft sind pflanzliche Arzneimittel und deren Wirkung.

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