LBSAT311 Rhabdomyolysis: A Rare Presentation Of Hashimoto Thyroiditis In An Adolescent Boy

Abstract

Introduction: Hashimoto thyroiditis is the most common cause of hypothyroidism and is associated with a wide range of clinical presentations in children and adolescents. Hypothyroidism-induced rhabdomyolysis without precipitating factors is extremely rare, particularly in pediatric patients. We describe a previously healthy adolescent boy who came to our institution with vague symptoms and was found to have rhabdomyolysis secondary to hypothyroidism due to Hashimoto thyroiditis.

Clinical Case: A 16 year old boy presented to the emergency department at Riley Hospital for Children due to a two week history of bilateral eye and lip swelling. Additional symptoms included fatigue, sleepiness, slowing of speech, decreased attention span and weight gain. He was not on any medications and had no history of allergies. There was no family history of endocrine or neuromuscular disorders. Physical exam revealed a height of 177 cm (75th tile), weight of 118.4 kg (99th %tile) and BMI at the 99th %tile. Exam was significant for slow generalized movements, facial edema, and delayed deep tendon reflexes in all extremities. The thyroid gland was normal. Laboratory evaluation revealed Creatinine-1.46 mg/dL (0.3-1.2), ALT-88 Units/L (7-52), AST-254 Units/L (13-39), TSH-32.2 mcU/mL (0.4-4.2), fT4 <0.2 ng/dL (0.6-1.5) and CK 20,500 Units/L (30-223). Thyroid peroxidase antibody was elevated at 686.1 IU/mL. A renal and bladder ultrasound showed trace intra-abdominal ascites and decreased echogenicity of the bilateral psoas muscles favoring edema and consistent with rhabdomyolysis. He received IV hydration and was started on levothyroxine 25 mcg daily that was increased over a 10 month period to achieve euthyroidism. Genetic testing for rhabdomyolysis revealed two variants of unknown significance. Acyl carnitine and carnitine profiles were normal ruling out a metabolic myopathy. His symptoms improved significantly after treatment with levothyroxine although his CK levels plateaued around 1,000 Units/L. Liver and kidney function normalized during follow up.

Conclusion: Rhabdomyolysis is a rare but serious complication of hypothyroidism, the pathogenesis of which is unclear. Proposed hypotheses include impaired mitochondrial oxidative metabolism, decreased muscle carnitine and switching of muscle fibers from fast-twitching type II to slow-twitching type I due to thyroxine deficiency ultimately leading to myolysis and release of intracellular muscle contents into the circulation. It is important for clinicians to have a high index of suspicion for hypothyroidism in children who present with muscle weakness and high creatinine kinase levels. The etiology of the disproportionately mild elevation of TSH in the setting of an unmeasurable free T4 and severe clinical hypothyroidism in our patient remains a mystery.