A case of thyrotoxicosis with prolonged muscle cramp and hypocalcemia after treatment with methimazole

: We report a case of thyrotoxicosis with prolonged post-treatment muscle cramp and hypocalcemia. A 36 year-old woman with hyperthyroidism was treated with Methimazole (MMI). As plasma levels of T4 and T3 were normalized, hypocalcemia was noted and severe cramp of skeletal muscle appeared so that the patient was unable to walk. The cramp was gradually relieved as the levels of thyroid hormones re-increased by discontinuance of MMI, and recurred as the hormone levels were normalized by readministration of MMI. The plasma levels of free calcium ion was positively correlated with those of thyroid hormones, and the muscle cramp was worsened with lowering of the calcium level. Serum examination also revealed vitamin D-deficiency, which was probably due to an unbalanced diet of the patient. A therapeutic trial with 1 alpha-vitamin D3 and calcium lactate in addition to MMI improved both thyrotoxicosis and muscle cramp. These findings suggested that hypocalcemia due to vitamin D-deficiency was involved in the exceptionally prolonged muscle cramp associated with the treatment of hypothyroidism in this patient.