RESUMO
Hypothyroidism is frequently associated with different neuromuscular disorders. However, myotonia is rarely a revealing feature. We report a case of hypothyroidism secondary to Hashimoto's thyroiditis and myotonia. The patient is a 45-year-old woman who consulted for a progressive myotonia. Blood and thyroid assessments revealed peripheral hypothyroidism with low free thyroxine, high thyroid-stimulating hormone levels and high titers of anti-thyroperoxidase antibodies. Outcome was favorable with thyroid hormone substitution. Hashimoto's thyroiditis is a common cause of hypothyroidism. The frequency of myopathy during hypothyroidism ranges from 30-80%. The main symptoms related are weakness, muscular cramps, and myalgia. Myotonia may reveal hypothyroidism. The pathogenic mechanism of this myotonia is still unknown. Good outcome of hypothyroid manifestations with thyroid hormone substitution requires systematic thyroid hormone screening in patients presenting with neuromuscular manifestations