Per case study data published in BMJ Case Reports, secondary proximal myopathy can occur with hypothyroidism and resolves with thyroxine replacement. The study investigators recommended that clinicians measure thyroid function in patients presenting with proximal myopathy.

The investigators described a 30-year-old patient who presented to her general practitioner with a 3-month history of global muscle weakness, frequent muscle spasms, and myalgia. Clinical examination of the respiratory system, cardiovascular system, abdomen, and extremities suggested no abnormalities. However, her creatine kinase level was elevated to 5124 U/L. Even though hypothyroidism was suspected from her clinical presentation, she was also screened for known viral causes of myopathy, including cytomegalovirus, Epstein-Barr virus, and hepatitis B and C. Serum cortisol, serum prolactin, serum vitamin B12, and 25-hydroxyvitamin D were also measured. Results from rheumatologic screening tests and a liver autoimmune profile were normal.

Laboratory tests revealed significantly elevated thyroid stimulating hormone (190 mU/L [range, 0.35-4.7 mU/L]), free thyroxine (<5 pmol/L [range, 7.8-21 pmol/L]), free triiodothyronine (2.7 pmol/L [range, 3.8-6.0 pmol/L]), and thyroid peroxidase antibodies (>1000 U/mL [range, 0-75 U/mL]). The patient was diagnosed with severe hypothyroidism due to Hashimoto thyroiditis. Intravenous fluids were administered and treatment was initiated with levothyroxine replacement for hypothyroidism. Within 4 months, the patient’s creatine kinase levels were in the normal range and the proximal myopathy completely resolved.

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The mechanisms by which Hashimoto thyroiditis has an impact on muscle function warrant further study. The investigators hypothesized that hypothyroidism may cause myofiber degeneration and reduced clearance of the creatine kinase enzyme.

At present, no medical guidelines exist regarding the investigation of proximal weakness and raised creatine kinase. However, based on the data from this case study, the investigators recommended routine examination of thyroid status and function in patients presenting with either myopathy or raised creatine kinase levels.

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Reference

Jervis W, Shah N, Mongolu SK, Sathyapalan T. Severe proximal myopathy secondary to Hashimoto’s thyroiditis. BMJ Case Rep. 2019;12:e230427.