Many thyrotoxic patients can be controlled with a single daily dose (SDD) of a thioamide. Until now, prediction of the effectiveness of a SDD has been impossible, and an empirical trial has been necessary. We have utilized a perchlorate discharge test to predict in advance the individual dose schedule required to control thyrotoxicosis. In 62 hyperthyroid patients studies with a perchlorate discharge test at 12 and 20 hr after a thioamide dose, we have been able to delineate 3 dose schedule groups. When tested at 12 hr, patients with discharges of less than 18% require a dose every 8 hr. Those with discharges from 18–51% can be controlled on a dose every 12 hr. Patients with discharges more than 69% require only a SDD. Patients with intermediate discharges of 51–69% were re-tested at 20 hr. Control can be achieved on a SDD if the discharge at 20 hr is greater than 25%. In our study, 68% of the patiients were controlled on a SDD, 22% on an every 12-hr dose, and 10% on an every 8-hr dose. In addition, a retrospective analysis suggests that methimazole is more effective than PTU in achieving control on the less frequent dose schedules.