Methimazole (MMI) is an anti-thyroid drug that belongs to drug class thionamides; the FDA approved uses of which include:
The non-FDA approved use of MMI includes treating thyrotoxicosis/thyroid storm.
The chief mechanism of action of methimazole is to block the production of thyroid hormone from the thyroid gland. It interferes with the step that causes iodination of tyrosine residues in thyroglobulin, mediated by the enzyme thyroid peroxidase, thus preventing the synthesis of thyroxine (T4) and triiodothyronine(T3). An additional mechanism is by inhibiting the iodotyrosyl residues from the coupling. Methimazole may also interfere with the oxidation of the iodide ion and iodotyrosyl groups. Eventually, thyroglobulin gets depleted, and circulating thyroid hormone levels decrease. It may also help to control diseases by affecting the overall immune system. Various studies show that reduction of immune molecules like intracellular adhesion molecule 1, soluble interleukin 2, and anti-thyrotropin receptor antibody over time, thus ameliorating immune-related hyperthyroid issues. Whether or not the improvements in the patient profile are due to this, or because of improvement of thyroid function, remains unclear.
However, there is no effect of this drug on the existing thyroxine (T4) and triiodothyronine (T3) in the circulation or stored in the thyroid gland. Similarly, there have been no observations of alterations in the effectiveness of exogenously administered thyroid hormones.
Methimazole administration is via the oral route. The starting dose is between 20 to 40 mg per day, depending upon the severity.
The treatment of thyroid storm includes a starting dose of 60 to 80 mg/day orally until achieving control; also given at 8-hour intervals. Adjust the subsequent doses and duration of treatment as per patient response.
Methimazole has a narrow therapeutic window. Therefore it is essential to note the maximally allowed dosage :
The side effects of methimazole are mostly dose-related. The minor ones like (most commonly) hives and itching, improve with anti-histaminic medications or by discontinuing the drug.
Serious adverse effects include:
Methimazole is contraindicated if there is hypersensitivity to the drug or any of its components.
The common symptoms of methimazole overdosage are nausea, vomiting, epigastric discomfort, fever, joint pain, itching, body ache, and swelling.
In cases of a drug overdose, initiate supportive therapy as per the patient's condition.
Physicians, nurses, and pharmacists in many parts of the world continue to use methimazole because of its effectiveness, and low cost for treatment of hyperthyroidism (mainly for Graves disease).
It is essential to know the side effects of methimazole, particularly severe drug allergy when taken with multiple medications, and side effects with the use of any thioamide medication in general. Furthermore, it is imperative to counsel the patient about the rare side effects like agranulocytosis or liver failure before starting the medication.
In general, methimazole prescribing should be from an endocrinologist, with patient monitoring by the primary care provider and nurse practitioner. Dose changes must not occur without first consulting with the endocrinologist. The pharmacist should verify all dosing, perform mediation reconciliation, and report any concerns back to the healthcare team. Nursing can verify medication compliance along with the pharmacist, as well as observe for any adverse effects. Only with open communication with members of the interprofessional team can the outcomes be improved and the adverse effects of the drug reduced. [Level V]
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