Introduction: Hypothyroidism is frequently associated with different neuromuscular disorders. However myotonia is rarely a revealing feature.
Clinical cases: Two patients aged 28 and 31 years consulted for a progressive myotonia. Blood and thyroid analysis revealed peripheral hypothyroidism with low FT4 and high TSH levels. Outcome was favorable with thyroid hormone substitution. Myotonia regressed and thyroid hormone levels returned to normal.
Conclusion: Myotonia may reveal hypothyroidism. The pathogenic mechanism of this myotonia is an unknown. Good outcome with thyroid hormone substitution requires a systematic thyroid hormone screening in patients presenting neuromuscular manifestations.