Myopathy Associated with Acute Hypothyroidism following Radioiodine Therapy for Graves Disease in an Adolescent

Graves Disease (GD) is uncommon in children with an estimated incidence of 1 per 10,000 [1]. GD accounts for 95% of the cases of hyperthyroidism in children and is more frequent in those with a familial history of autoimmune thyroid disease [2]. Although pharmacological therapy with antithyroid drugs (ATDs) is the first-line therapy in pediatric GD, long-term remission rates of GD in children are less than 25% [3]. Thus, definitive treatment with radioactive iodine (RAI, I-131) or surgery is required for the majority of individuals with GD [3, 4]. When I-131 is used, the treatment goal is hypothyroidism which typically occurs 2–4 months after the administered dose [5, 6].Muscle abnormalities can occur in hypothyroidism via mechanisms that are unclear [7]. Hypothyroidism-related myopathy has been observed in adults and children [8], with a wide spectrum of problems including myalgias, proximal myopathy, and muscle hypertrophy [7]. Elevation of serum creatine kinase (CK), associated with rhabdomyolysis and renal complications, has been reported [9–12]. Fifty seven to 90 hypothyroid individuals can have CK elevations [13, 14], a phenomenon not observed in hyperthyroidism [15].Acute myopathy with onset during the acute hypothyroidism following treatment of GD is rare in adults [16]. Severe rhabdomyolysis, a more severe state where elevation of muscle enzymes is additionally associated with myoglobinuria, elevated creatinine, and renal failure, has also been observed in this setting [17, 18]. In children, myopathy with hypothyroidism is rare [19], and we are unaware of reports of acute myopathy following 131-I treatment in the pediatric population. We now describe acute myopathy following I-131 treatment for hyperthyroidism due to GD in an adolescent.A 15-year-old male was diagnosed with GD. At presentation, there was goiter (estimated 40–60 grams of thyroid tissue). The total thyroxine (T4) was 27.8？mcg/dL (357.8？nmol/L SI) (normal 4.5–12.5？mcg/dL), estimated free thyro