A DIAGNOSTIC RADIOIODINE UPTAKE TEST IN PATIENTS RECEIVING ANTITHYROID DRUGS*

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Abstract
In 74 patients uninfluenced by thyroid-affecting medication, the fraction of an intravenously injected tracer dose of I131 present in the thyroid region ten minutes after injection (N5–15) was calculated and plotted against the one-hour uptake rate. The curve thus obtained was used as a calibration curve in subsequent studies. Forty-one patients were studied under two or more of the following regimens: (i) untreated; (ii) receiving a full course of antithyroid drug treatment; (iii) receiving a thyroid hormone preparation in “suppressive” dosage; (iv) receiving a combination of (ii) and (iii). Multiplying the ten-minute thyroidal I131 uptake rate in a patient receiving antithyroid drugs by a factor of 1.22 yields the basic untreated one-hour uptake rate. The ten-minute uptake rate is suppressible by a thyroid hormone preparation in nonthyrotoxic patients but is not suppressible in thyrotoxic patients. INTRODUCTION MOST tests used clinically for the determination of thyroid function by measurement of the radioiodine uptake rate are temporarily invalidated by the prior administration of an antithyroid drug. It is necessary to discontinue the administration of the drug long enough before the test for the blocking effect to be dissipated, and sometimes a thyroid hormone preparation must be given for from one to three weeks to distinguish thyrotoxic high uptake from nonthyrotoxic high uptake (1, 2). During this period of cessation of antithyroid drug therapy, severe clinical manifestations may develop in some thyrotoxic patients. Even when such an untoward development does not occur, the delay in instituting definitive treatment may be inconvenient for both the patient and the physician.

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