Endocrine Abstracts

JOINT1941

Antithyroid arthritis syndrome (AAS) is a rare but significant adverse reaction linked to antithyroid therapy. We present a case of an 8-year-old girl with celiac disease and newly diagnosed Graves’ disease who developed arthritis and systemic symptoms 22 days after starting methimazole (5 mg three times a day; 0,46 mg/kg). The patient presented with arthralgia and swelling of both wrists and the third finger of the right hand. She also had difficulty walking due to right hip pain. She reported a one-week history of an urticarial rash on her trunk and upper limbs, along with systemic symptoms, including diarrhea, abdominal pain, vomiting, and loss of appetite. During hospitalization, she developed a fever. Laboratory tests showed a normal complete blood count, elevated inflammatory markers (CRP 2.68 mg/dl, ESR 31 mm/h), and mildly elevated liver enzymes. Physical examination revealed tenderness on flexion-extension of both wrists (worse on the right), swelling of the right wrist, pain on mobilization of the right hand’s proximal interphalangeal JOINT, and tenderness with limited range of motion of the hip JOINT. The rheumatologist confirmed arthritis. Microbiological tests were negative. A JOINT ultrasound confirmed bilateral synovitis. Due to persistent fever, whole-body CT and bone scintigraphy were performed and were negative. As thionamide arthritis was suspected, methimazole therapy was reduced to 5 mg twice daily (0,31 mg/kg) and ibuprofen (300 mg three times daily) was administered for 10 days, followed by a further 10 days as required. The fever resolved on the 10th day of hospitalization and the patient was discharged; after 20 days the patient had no further clinical signs of arthritis and laboratory tests were completely normalized. The patient subsequently discontinued thionamide therapy after 6 months from the start, with normal thyroid function tests and negative TSH-receptor antibodies. The patient remained stable with no recurrence of arthritis or systemic symptoms. The close timing between methimazole initiation and the onset of arthritis and systemic symptoms and the exclusion of other causes, strongly suggest AAS. This case highlights the importance of recognizing this rare adverse drug reaction in pediatric patients receiving antithyroid therapy. To the best of our knowledge, this is the first case of AAS in pediatric patients, probably due to an underestimation of this side effect and the rarity of Basedow’s disease in pediatrics. Early recognition and reduction of thionamide are crucial for resolution of symptoms, highlighting the need for vigilance when prescribing in pediatric populations.