Pitfalls of Levothyroxine Replacement Therapy
Hypothyroidism (reduced function of the thyroid gland) affects approximately 1-2% of the population. Its prevalence increases with age and it is more common in women. Synthetic thyroid hormone levothyroxine is used to treat hypothyroidism. It is a widely prescribed medication available in tablets of various strengths.
Therapeutic Window of Levothyroxine
Levothyroxine is a drug with a narrow therapeutic window, and correct dosage adjustment plays a crucial role. Caution is needed when treating patients with cardiovascular diseases, pregnant women, the elderly, and others sensitive to even small changes in thyroid hormone levels. The bioavailability of levothyroxine is influenced by several factors, including the content of levothyroxine in different preparations as well as concurrently consumed foods, medications, and beverages.
It has been previously demonstrated that the content of the active ingredient decreases near the end of the shelf life in some preparations. The shelf life varies among levothyroxine products, typically ranging between 2-3 years. Rules for the content of active substances in products with narrow therapeutic windows, including levothyroxine, have been gradually tightened. Initially in the USA, and later in the United Kingdom, New Zealand, and Denmark, the rules for the active substance content throughout the product's shelf life were newly set to 95-105% (previously 90-110%).
Stability of Euthyrox
In response to this legislative change, Merck modified the excipients of its medicinal product (replacing lactose with mannitol and adding citric acid). Euthyrox now has a more accurate and stable active ingredient content, is bioequivalent to other market products, and meets stricter criteria. Thanks to higher stability, Euthyrox guarantees an active ingredient content of 95-105% throughout the shelf life (3 years). Additionally, the medication can be used by patients with lactose intolerance.
Interchangeability of Levothyroxine Products
European Thyroid Association (ETA) and Thyroid Federation International (TFI) in a joint statement on the interchangeability of levothyroxine in EU countries state that switching from one product to another can cause patients issues with disease compensation and side effects. Therefore, ETA and TFI recommend treating patients with the same levothyroxine product whenever possible. If switching is necessary, proper dosage adjustment is required again.
(epa)
Sources:
1. Gabalec F. Greater stability of the new Euthyrox formulation? Why was the change needed? DMEV 2019; 22 (3): 127−129.
2. Kršek M. Euthyrox with new formulation. 42. Endocrinology Days, Olomouc, October 10, 2019.
3. Lipp H.-P., Hostalek U. A new formulation of levothyroxine engineered to meet new specification standards. Curr Med Res Opin 2019; 35 (1): 147−150, doi: 10.1080/03007995.2018.1545635.
4. SPC and package leaflet Euthyrox 100 μg. Available at: www.sukl.cz/modules/medication/detail.php?code=0243130&tab=texts
5. Suchopár J., Prokeš M. Levothyroxin – Originals, generics, and their interchangeability. DMEV 2020; 23 (3): 119−126.
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