Hypothyroidism unmasking proximal myotonic myopathy.

No specific diagnostic test is available to identify patients with proximal myotonic myopathy and to distinguish them from common disorders causing similar complaints. We describe three patients from three separate families who were initially diagnosed as having hypothyroid myopathy. Proximal weakness, stiffness and myotonia have persisted in each patient (2-10 years) despite the restoration of the euthyroid state. A familial pattern of autosomal dominant inheritance for proximal weakness, myotonia, and cataracts was clearly identified in one family and was likely in the other two families. DNA testing showed normal size of CTG repeat in the gene for myotonic dystrophy. The clinical presentation of these three patients strongly suggests that hypothyroidism can unmask PROMM in asymptomatic individuals who carry the genetic abnormality. Other cases of 'hypothyroid myopathy' may represent examples of unmasked PROMM.