Polyarthritis caused by methimazole in two Japanese patients with graves' disease

Area of Science:

• Pediatric Endocrinology
• Rheumatology
• Clinical Pharmacology

Background:

• Graves' disease (GD) is commonly treated with methimazole (MMI) in children.
• Antithyroid drugs (ATDs) can cause side effects, ranging from 6% to 35% in pediatric patients.
• Polyarthritis is an uncommon but serious ATD-induced side effect, potentially linked to antineutrophil cytoplasmic antibody-associated vasculitis.

Observation:

• Two pediatric patients with GD developed polyarthritis within a month of starting MMI therapy.
• Symptoms of polyarthritis rapidly resolved after MMI was discontinued.
• Alternative treatments like radioactive iodine (131I) therapy or thyroidectomy were used post-MMI cessation.

Findings:

• Methimazole (MMI) therapy was identified as the likely cause of severe polyarthritis in pediatric Graves' disease patients.
• Discontinuation of MMI led to rapid symptom resolution, confirming the drug as the etiological agent.
• The occurrence, though rare, highlights MMI-induced polyarthritis as a significant adverse event in pediatric GD.

Implications:

• Severe polyarthritis is a critical side effect of methimazole in pediatric Graves' disease, necessitating prompt treatment cessation.
• Early recognition and withdrawal of MMI are vital for managing this adverse reaction.
• This case series underscores the importance of monitoring for rheumatological side effects during MMI therapy in children.