Pharmacological properties

The synthetic levothyroxine contained in Euthyrox is identical in effect with the naturally occurring major hormone secreted by the thyroid.

After absorption it is largely bound to plasma proteins, particularly thyroxine-binding globulin (TBG). It passes very gradually into the cells of the body where, after partial conversion to liothyronine (= L-triiodothyronine), it develops the characteristic thyroid hormone effects on development, growth and metabolic systems.

Acute toxicity:

Levothyroxine has a very slight acute toxicity. In incidents of poisoning (attempts at suicide) in humans, doses of 10 mg levothyroxine were tolerated without complications. In general, serious complications, such as a threat to vital functions (respiration and circulation) need not be anticipated unless coronary heart disease is present.

Chronic toxicity:

The chronic toxicity of levothyroxine was studied in various animal species (rat, dog). At high doses, signs of hepatopathy, increased occurrence of spontaneous nephroses as well as changes in organ weights were observed in rats. No substantial adverse reactions occurred in dogs. Several cases of sudden cardiac death have been reported in patients with long years of levothyroxine abuse.

Reproduction toxicity:

Thyroid hormones pass the placenta only in slight, ineffective amounts. After injection of small doses (up to 24 µg) of levothyroxine in chicken embryos the incidence of strophosomia was increased. Ample experience in humans is available on therapy with levothyroxine in all phases of pregnancy: there is no evidence of any toxic effect on the fetuses or of drug-induced malformations.

No information is available on harmful effects on male or female fertility. There have been no indications of any kind in this connection.

Mutagenicity

No information is available on this subject. So far no indications of any kind have become known suggesting damage to the progeny due to changes in the genome caused by thyroid hormones.

Carcinogenicity

No long-term animal studies have been carried out with levothyroxine.