Long-standing recurrent paroxysmal supraventricular tachycardia disappearing after cystic degeneration of an episodically hypersecreting toxic adenoma of the thyroid.

A 35-year-old woman with an otherwise normal heart had been suffering from recurrent paroxysmal supraventricular tachycardia during the past 3 years. Serum thyroid function tests were repeatedly normal. When a thyroid nodule became evident, a toxic adenoma was diagnosed. Serum total hormone values were normal, but a few hours after the last recurrence of paroxysmal supraventricular tachycardia, total T3, free T3 and free T4 became clearly elevated. At that time hemorrhagic necrosis of the adenoma, though localized only at the periphery of the tumor, heavily impaired nodule overactivity, as judged by the disappearance of paroxysmal supraventricular tachycardia, by normal free thyroid hormone levels and by the return of a normal TSH response to TRH.