Case Presentation: A 50-year-old Latin-American woman with a history of hyperthyroidism presented to the ED with migrating polyarthralgias and fever for several days. 3 months prior, patient began experiencing stiffness, joint pains, fatigue, and palpitations. She was ultimately diagnosed with hyperthyroidism and started on Methimazole 1 month prior to current presentation. Patient presented to the ED after noting that joint pain initially present on right shoulder migrated over the next several days. The patient also described a blotchy erythematous rash on her chest, neck and arms that temporarily resolved with hydrocortisone cream 1 week prior to admission. Family history was significant for her mother having Rheumatoid Arthritis. She denied sexual activity, sick contacts, recent travel, or known tick bites. On admission, her temperature was found to be 102 degrees Fahrenheit. Her exam was remarkable for right wrist erythema and swelling. She also displayed decreased range of motion in her shoulders, hips and knees secondary to pain. Labs showed a mildly elevated ESR to 32 and CRP elevated to 19.9, no leukocytosis and negative cultures. During hospitalization, the patient’s right wrist swelling resolved, but left wrist began to swell. She also developed a new urticarial rash that resolved with topical steroids. Further lab work was positive for Rheumatoid Factor however, she maintained normal complement levels. As subsequent autoimmune and infectious workup was negative, there was a growing concern for a serum sickness reaction due to the temporal relationship with Methimazole starting about 1 month prior to symptoms. Methimazole was stopped and she was started on atenolol for symptomatic management of hyperthyroidism. At her 2 week follow up, her joint pain had improved and her ESR and CRP normalized. She had radioactive iodine thyroid ablation 1 month after discharge and her symptoms have completely resolved.
Discussion: Serum sickness is a rare but known side effect of several different medications, one of which is Methimazole. Its cardinal features include rash, fever, polyarthalgias/polyarthritis. These symptoms often begin 1-2 weeks after exposure to the medication responsible and improve after discontinuation as this clinical syndrome is a type III/immune complex mediated hypersensitivity disease. The diagnosis is usually a clinical one after exclusion of other potential etiologies. Complement levels may be depressed during severe episodes however, levels do not need to be depressed as in this case. There have been a few reported cases of Methimzaole induced serum sickness. In one case, a 15-year-old boy was admitted to the hospital with joint pain and swelling of his ankles and knees after starting Methimazole 3 weeks prior for hyperthyroidism. Symptoms resolved upon stopping Methimazole [1]. Another case reports a female patient with multinodular goiter who was treated with methimazole for thyrotoxicosis and developed sore throat, pruritus, arthralgias and fever after 3 weeks of starting that drug [2]. While acute migratory polyarthritis with fever is typically caused by viral arthritis, disseminated gonococcal infection or rheumatic fever, medication induced serum sickness should be considered as evidenced by this case and previous case reports.
Conclusions: Serum sickness is a known but rare side effect of Methimazole, a very commonly prescribed medication. This case is a prime example to always consider medication side effects as potential etiologies of a patient’s symptoms.