From the Research
Tapazole (methimazole) should be discontinued immediately if a patient develops significant eosinophilia (>1,500 cells/μL) or eosinophilia accompanied by symptoms like rash, fever, or organ dysfunction, as it can be a potential cause of the condition. Eosinophilia is a known side effect of Tapazole, occurring in 2-5% of patients, and may be part of a hypersensitivity reaction 1. The mechanism involves Tapazole triggering an immune response that increases eosinophil production. If mild and asymptomatic, close monitoring may be sufficient while continuing the medication. However, alternative treatments for hyperthyroidism, such as propylthiouracil (PTU), radioactive iodine therapy, or surgery, should be considered if Tapazole is discontinued.
According to the World Health Organization-defined eosinophilic disorders, the goal of therapy is to mitigate eosinophil-mediated organ damage, and corticosteroids are first-line therapy for patients with lymphocyte-variant hypereosinophilic syndrome (HES) and idiopathic HES 2. The use of interleukin-5 (IL-5) antagonists, such as mepolizumab, may also be considered in the treatment of HES.
In terms of monitoring, regular complete blood counts are recommended for patients on Tapazole, especially during the first few months of treatment, to detect potential side effects such as agranulocytosis, a rare but serious condition that requires immediate medical attention. A study comparing methimazole and propylthiouracil in patients with hyperthyroidism caused by Graves' disease found that methimazole 15 mg/d is suitable for mild and moderate cases, whereas methimazole 30 mg/d is advisable for severe cases 3.
Key points to consider:
- Tapazole can cause eosinophilia as a side effect in some patients
- Eosinophilia typically occurs in 2-5% of patients on Tapazole and may be part of a hypersensitivity reaction
- If mild and asymptomatic, close monitoring may be sufficient while continuing the medication
- If significant or accompanied by symptoms, Tapazole should be discontinued immediately and alternative treatments considered
- Regular complete blood counts are recommended for patients on Tapazole, especially during the first few months of treatment.