Levothyroxine and the Interchangeability of Preparations Containing It
Levothyroxine is a synthetic thyroid hormone used to treat patients with hypothyroidism, which affects approximately 1-2% of the younger population and increases with age, especially in women. Levothyroxine is a drug with a narrow therapeutic window, and its bioavailability is influenced by many factors, including consumed foods, medications, and beverages.
Originals, Generics, and Their Substitution Possibilities
As of July 2020, there were 10 medicinal preparations containing varying amounts of levothyroxine registered in the Czech Republic, which cannot be automatically considered interchangeable, even though Czech legislation permits it. When registering a drug, original preparations are considered substances with their own complete registration documentation, while generics are preparations without their own complete registration documentation, which demonstrate so-called bioequivalence with a reference drug during registration (generally 80–120%, for drugs with a narrow therapeutic window 90–110%). The reference drug can be either an original or a generic preparation.
In the case of levothyroxine, Euthyrox, Eltroxin, and Letrox are original preparations with their own complete documentation, and their mutual bioequivalence has therefore not been studied. Syntroxine is a partial generic to Euthyrox (it lacks its own complete documentation and refers to Euthyrox as the reference drug when demonstrating bioequivalence but has different bioavailability and varies in terms of diet influence).
Position of Professional Societies
It is thus no surprise that the European Thyroid Association (ETA) and the Thyroid Federation International (TFI), in a joint statement on the interchangeability of levothyroxine in EU countries, state that switching one preparation for another can cause patients issues with disease compensation and adverse effects. Similar problems previously occurred in Denmark, the Netherlands, and France. ETA and TFI therefore recommend treating patients with the same levothyroxine preparation whenever possible. If a change is necessary, it is appropriate to evaluate TSH and FT4 levels after 4-6 weeks and adjust the levothyroxine dose if necessary.
Prescription Labeling
As mentioned, in the Czech Republic, drugs containing levothyroxine are interchangeable under generic substitution, even though it is not advisable for the reasons stated above. If the prescribing doctor wants to ensure the patient receives the specifically prescribed preparation, it is advisable to use the “do not substitute” label on the prescription.
(epa)
Source: Suchopár J., Prokeš M. Levothyroxine – Originals, Generics, and Their Interchangeability. DMEV 2020; 23 (3): 119–126.
Did you like this article? Would you like to comment on it? Write to us. We are interested in your opinion. We will not publish it, but we will gladly answer you.
