An outbreak (159 cases) of transient thyrotoxicosis without hyperthyroidism in Japan.

We report an outbreak of transient thyrotoxicosis that occurred between June and September 1993, in Matsuyama City, Ehime Prefecture. One hundred fifty-nine cases of thyrotoxicosis were identified, all in individuals without goiter. Thyroid autoantibodies were absent, and the serum thyroglobulin level was depressed. Ultrasonography of the thyroid disclosed no abnormal findings, while scintigraphy of the thyroid with 99mTc revealed poor uptake. About ten days after the onset, the serum thyroid hormone levels were normalized. No significant elevation of serum viral antibodies was found. These observations suggest that a thyrotoxicosis factitia may have been the cause of the present outbreak.