Generic prescribing of levothyroxine remains appropriate for the majority of patients, the MHRA says, and this approach is supported by strict UK regulatory requirements for licensing to ensure bioequivalence between products.
However, a small proportion of patients treated with levothyroxine report symptoms, often consistent with thyroid dysfunction, when their levothyroxine tablets are changed to a different brand or manufacturer's product.
In this situation, the MHRA suggests testing the patient's thyroid function. If symptoms persist, it says healthcare professionals should consider consistently prescribing a specific product known to be well tolerated by the patient, even if they are biochemically euthyroid.
If symptoms or poor control of thyroid function still persist despite adhering to a specific product, the MHRA suggests prescribing levothyroxine in an oral solution formulation.
Yellow card reports
For the 5-year period between 1 January 2015 and 31 December 2019, the MHRA received 335 Yellow Cards reporting one or more of the terms ‘product substitution issue’, ‘condition aggravated’ or ‘drug ineffective’ with levothyroxine. The majority of reports were received from patients and the symptoms reported were mostly consistent with hypothyroidism or hyperthyroidism.
Treatment guidance from the British Thyroid Association acknowledges symptoms can occur in some people on switching. For the most part they could indicate the need for dose adjustment, although some patients still experience symptoms despite tests showing they are biochemically euthyroid.
The underlying causes for the symptoms are generally unclear. It has been suggested that causative factors could include reduced levothyroxine absorption (eg, due to gastrointestinal comorbidities), very low thyroid reserve or intolerance or allergy to a particular excipient.