Thyrotoxic Myopathy Mimicking Myasthenic Syndrome Associated with Thymic Hyperplasia

Emiko Tsuda; Tomihiro Imai; Akihiro Matsumura; Shin Hisahara; Michio Nonaka; Hirokazu Shiraishi; Masakatsu Motomura; Shun Shimohama

doi:10.2169/internalmedicine.47.0682

CASE REPORTS

Thyrotoxic Myopathy Mimicking Myasthenic Syndrome Associated with Thymic Hyperplasia

Emiko Tsuda, Tomihiro Imai, Akihiro Matsumura, Shin Hisahara, Michio Nonaka, Hirokazu Shiraishi, Masakatsu Motomura, Shun Shimohama

Author information

Emiko Tsuda
Department of Neurology, Sapporo Medical University School of Medicine
Tomihiro Imai
Department of Neurology, Sapporo Medical University School of Medicine
Akihiro Matsumura
Department of Neurology, Sapporo Medical University School of Medicine
Shin Hisahara
Department of Neurology, Sapporo Medical University School of Medicine
Michio Nonaka
Department of Neurology, Sapporo Medical University School of Medicine
Hirokazu Shiraishi
The First Department of Internal Medicine, Graduate School of Biomedical Science, Nagasaki University
Masakatsu Motomura
The First Department of Internal Medicine, Graduate School of Biomedical Science, Nagasaki University
Shun Shimohama
Department of Neurology, Sapporo Medical University School of Medicine

Keywords: hyperthyroidism, Graves' disease, thyrotoxic myopathy, myasthenia gravis, Lambert-Eaton myasthenic syndrome, thymic hyperplasia

JOURNAL OPEN ACCESS

2008 Volume 47 Issue 5 Pages 445-447

DOI https://doi.org/10.2169/internalmedicine.47.0682

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Abstract

A 41-year-old man with progressive limb weakness manifested fluctuating muscle weakness as seen in myasthenia gravis (MG). Laboratory investigations revealed hyperthyroidism without the complication of MG. Electrophysiological studies demonstrated abnormal features of neuromuscular transmissions resembling those of the Lambert-Eaton myasthenic syndrome rather than those of MG. A CT scan showed a mediastinal mass that suggested thymic hyperplasia which often complicates MG or hyperthyroidism. Medical treatment of hyperthyroidism resulted in resolution of MG-like symptoms and regression of thymic hyperplasia on CT concomitant with normalization of thyroid function. This case highlights the fact that careful investigations are needed to differentiate MG-like symptoms from genuine MG in cases of hyperthyroidism with thymic lesions.

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