Myasthenia gravis in conjunction with Graves' disease: a diagnostic challenge.

OBJECTIVE: To describe an association between Graves' disease and myasthenia gravis and discuss the clinical features and laboratory tests that may help distinguish these two diseases.

METHODS: The clinical, laboratory, and electrophysiologic findings in a patient with Graves' disease and myasthenia gravis are presented.

RESULTS: A 28-year-old African American man was admitted to the University of Louisville Hospital with generalized muscle weakness, exophthalmos, diplopia, weight loss, and mild dysphagia. The diagnosis of Graves' disease with ophthalmologic involvement was suspected clinically and confirmed by an undetectable thyrotropin level (<0.03 mIU/mL), high total thyroxine (20.5 mg/dL), and increased homogeneous 123I thyroid uptake. Because of the generalized muscle weakness and mild dysphagia, assessment was done by a neurology team, and severe thyrotoxic myopathy was diagnosed. He was treated with 131I and b-adrenergic blocking agents and scheduled for follow-up as an outpatient. Two weeks later, the patient presented in acute respiratory failure. The neurology team was reconsulted because of suspected myasthenic crisis. Anti-acetylcholine receptor antibodies were undetectable in the serum, and computed tomography of the chest showed no thymic enlargement. Repetitive nerve stimulation testing, however, showed findings consistent with an abnormality of the neuromuscular junction. The patient responded dramatically to an anticholinesterase agent and corticosteroids.

CONCLUSION: The overlapping clinical features may cause diagnostic confusion when myasthenia gravis and Graves' disease coexist, and numerous tests may be needed to distinguish these two conditions, which have differing treatments and prognoses.