Asymptomatic myotonia congenita unmasked by severe hypothyroidism

Myotonia congenita is an inherited muscle disorder sustained by mutations in the skeletal muscle chloride channel gene CLCN1. Symptoms vary from mild to severe and generalized myotonia and worsen with cold, stressful events and hormonal fluctuations. Here we report the case of a young woman who sought medical attention because of subacute onset of diffuse and severe limb myotonia. CLCN1 gene sequencing showed a heterozygous transversion (T550M), two polymorphisms and one silent mutation. Thyroid function screening revealed severe hypothyroidism. She was placed on l-thyroxine replacement therapy which dramatically improved myotonia. We conclude that hypothyroidism unmasked a genetically determined, clinically asymptomatic chloride channelopathy. Diagnostic work-up in patients with clinically isolated myotonia should not be limited to genetic screening of non-dystrophic or dystrophic myotonias. Considering the high prevalence of hypothyroidism in females, systematic thyroid function screening by looking for additional hypothyroid symptoms and serum TSH levels measurement is mandatory in these patients.