Endocrine Abstracts

Introduction: Thyrotoxicosis as well as its treatment with Carbimazole (CBZ) or Propylthiouracil (PTU) have been reported to result in myopathy. Onset of myalgia during the treatment of thyrotoxicosis may cause a diagnostic challenge in distinguishing the thyrotoxic from the drug-induced myopathy. We report such a case where myopathy was induced by treatment with both CBZ and PTU.

Case report: We report the case of a 26-year-old Chinese lady presenting with palpitations due to Graves’ thyrotoxicosis (TSH <0.01 mIU/l, Free T4 52.4 pmol/l, TSH receptor antibodies 7.1 IU/ml, and Anti-Thyroid Peroxidase antibodies >1000 IU/ml). She developed myalgia, muscle cramps and weakness involving the upper and lower limbs with significantly elevated Creatine Kinase (CK) of 2651 U/l (normal range 29 – 168 U/l) when she was commenced on CBZ 40 mg daily. The MRI scan of all limbs showed normal muscle bulk with no evidence of myositis. Although her free thyroid hormones settled with CBZ, her symptoms did not respond to reducing the dose to 20 mg daily. Changing the treatment to PTU 100 mg twice daily resulted in a transient improvement of symptoms and a reduction of the CK level. Her serum potassium, calcium, and renal functions were within normal ranges. The muscle cramps and myalgia recurred while on PTU in two weeks with high CK despite normal free thyroid hormones and completely suppressed TSH. Her TSH receptor antibody level remained elevated throughout the course of treatment. Since she had myopathy with both antithyroid agents, she has been referred for total thyroidectomy.

Conclusion: We report a patient with Graves’ thyrotoxicosis who developed myopathy during anti-thyroid drugs treatment. High CK and normal potassium levels are reported more frequently in drug-induced myopathy, unlike Grave’s myopathy which often presents with normal CK and hypokalemia. This alongside the sequence of symptom onset may help distinguishing between the two conditions.