Method In euthyroid patients, if thyroid activity-measured by standard counting techniques-after a tracer dose of" 3'I is plotted against the square root of the time in minutes, the resultant curve is seen to be a straight line persisting for up to 10 hours after the administered dose: drugs exhibiting antithyroid effect can cause deviations from this straight line, and are grouped by Stanley and Astwood into the following grades: 0, no effect; 1, questionable or slight effect; 2, partialsuppression of uptake; 3, complete inhibi-tion for less than 4 hours; 4, complete inhibition for more than 4 to 24 hours; and 5, complete inhibition for more than 24 hours.