Myositis-Like Syndrome Revealing Hypothyroidism

Introduction: Muscular involvement during hypothyroidism is rare and often infra-clinical. Symptomatic forms simulating authentic inflammatory myopathy remain exceptional and difficult to diagnose. We report a particular observation of primary hypothyroidism presenting initially by a “myositis-like” syndrome.

Case Report: A 66-year-old woman with no medical history was explored for myalgia associated with proximal muscular weakness, which had been evolving for the past month, with an increase in muscle enzymes: CPK at 1570 IU/l and LDH at 607 IU/l. She was sent to us for suspicion of polymyositis. The electromyogram was non-contributory and the muscle MRI was without abnormalities. Anti-nuclear antibodies and anti-soluble antigen antibodies were negative.
The thyroid assessment showed TSH at 15.45 μIU/ml, fT4 at 10.55 pmol/l, anti-thyroglobulin antibodies positive at 1000 IU/ml, and anti-thyroperoxidase positive at 488.5 IU/ml confirming the diagnosis of Hashimoto’s thyroiditis. Under thyroxine, the evolution was favorable with disappearance of muscular complaints and normalization of muscular enzymes.

Conclusion: The “myositis-like” syndrome is an exceptional presentation of hypothyroidism that represents a real diagnostic challenge for the clinician, especially since a true combination of thyroiditis and myositis is possible. It is recommended to screen for thyroid dysfunction in front of presentation suggestive of myositis, especially that this myosite-like syndrome may be the first inaugural manifestation of hypothyroidism.

Keywords: Myosite-like syndrome, hypothyroidism, CPK, myositis, thyroiditis.