Myasthenia gravis associated with Graves' disease in Japan.

The prevalence of myasthenia gravis in Graves' disease was 0.14% in the 22,956 patients with Graves' disease who came during 1968-1979 to the Ito Hospital, Tokyo. While age at the onset of Graves' disease in patients with myasthenia gravis was slightly lower than that in those with Graves' disease without myasthenia gravis, age at the onset of myasthenia gravis in patients with Graves' disease was not different from that of myasthenia gravis patients without Graves' disease. Of 33 patients with both myasthenia gravis and Graves' disease, 8 developed myasthenia gravis first, 13 developed Graves' disease first, and in 12 the two diseases occurred concurrently. Most patients in whom Graves' disease developed first had clinical manifestations of myasthenia gravis within 2 years of the onset of Graves' disease. The dosage of an anticholinergic drug required to control symptoms decreased as the thyroid function was normalized, and no cases showed the "see-saw phenomenon".