INTRODUCTION: Both myasthenia gravis (MG) and autoimmune thyroid diseases (AITDs) are autoimmune diseases. Graves'disease (GD) is the most common AITD reported to be associated with MG. Currently, there is limited data on prevalence and clinical features/outcomes of MG in various thyroid diseases in a large database report.

METHODOLOGY: A total of 872 patients with MG and 97,251 patients with thyroid disorders had been recorded by the tertiary hospital database. The study period was between 1997 and 2017. Patients with a thyroid disorder and MG were identified by the ICD-10-CM code. Clinical courses of MG accompanied by thyroid disorders were studied.

RESULTS: During the 20-year study period, there were 872 patients with MG and 97,251 patients with thyroid disorders. In the group with thyroid disorders, 28,886 patients (29.70%) had GD, 1,612 patients (1.66%) had Hashimoto's thyroiditis, 13,172 patients (13.54%) had toxic goiter and 53,581 patients (55.10%) had nontoxic goiter. 97 patients had been diagnosed with both MG and thyroid disorders. Among the four types of thyroid disorders, the rate of MG was highest in HT group (9.92/1,000 HT patients). There were four significant factors among four groups of thyroid disorders including age of onset of thyroid disease (p 0.004), MG classification (ppp 0.034). Among the four groups of thyroid disorders, patients with MG and HT were diagnosed with thyroid disease at the youngest age (27 years) compared with other thyroid diseases. Additionally, the MG patients with HT also had the highest proportion of MG class 4-5 a/b (7 patients, 43.75%), received prednisolone treatment (15 patients, 93.75%), received immunosuppressants (9 patients, 56.25%), received IVIG or PLEX (5 patients, 31.30%), and had thymoma (6 patients, 46.15%).

CONCLUSION: MG is most prevalent in patients with HT. Patients with both MG and HT had more severe MG status and had higher rate of thymoma.