Abstract
Hypothyroidism is a common clinical condition with nonspecific symptoms such as fatigue, cold intolerance, and constipation. Rarely, severe primary hypothyroidism presents with rhabdomyolysis. We present a 12-year-old boy with several months of fatigue, muscle cramping, and elevated creatine kinase (CK) who was found to have severe primary hypothyroidism. Initial laboratory evaluation was significant for CK 2056 U/L (reference, 0-300 U/L; 34.34 µkat/L) and creatinine 1.39 mg/dL (reference, 0.4-1 mg/dL; 122.88 µmol/L). He was admitted for management of rhabdomyolysis with acute kidney injury. Further biochemical testing revealed profound hypothyroidism-thyrotropin 494 mIU/mL (reference, 0.40-6.00 mIU/mL) and free thyroxine (T4) less than 0.4 ng/dL (reference, 0.80-1.80 ng/dL; <5.15 pmol/L). Thyroglobulin and thyroid peroxidase autoantibodies were positive, confirming autoimmune hypothyroidism. Low-dose levothyroxine was initiated. With aggressive rehydration, creatinine and CK levels improved. The patient was discharged home with instructions to escalate thyroid hormone replacement over 8 weeks. While the etiology of CK elevation in severe hypothyroidism is poorly understood, it is hypothesized that T4 deficiency alters mitochondrial oxidative capacity and glycogenolysis precipitating muscle atrophy and breakdown with CK release. This case highlights that clinicians should consider thyroid function testing in patients with symptoms of muscle pain and unexplained elevations in CK.