[Unmasking congenital myotonia by hypothyroidism]

A case with the combination of hypothyroidism and true myotonia is reported, in which the latter first became manifest clinically together with the thyroid disorder and improved with L-thyroxine therapy. The hypothyroidism itself caused very few symptoms, and the diagnosis was not made until examination for myotonia. The effect of each disorder on muscle function seems to be additive.