The T₃ suppression test is based on the finding that the administration of T₃ (75–100 μg per day for 8–10 days) suppresses the uptake of radioiodine in the normal thyroid gland by at least 50% but does not suppress uptake in Graves disease or toxic nodular goiter because the thyroid glands in the latter are autonomous and not dependent on pituitary TSH.56,57 The T₃ suppression test, once popular for the diagnosis of borderline hyperthyroidism, is only occasionally used today in conjunction with imaging to determine thyroid nodular autonomy. Modifications of the standard T₃ suppression test are necessary in some conditions. Occasional patients, particularly those with nodular thyroids, may exhibit autonomous increase in thyroid function while receiving long-term replacement therapy with T₄.58 This may be confirmed by measuring the thyroid uptake without discontinuing T₄ and without the addition of T₃. Such uptake measurements normally should be no more than 1% to 2%. The adequacy of suppressive thyroid hormone therapy can be evaluated in the same manner, although the ultrasensitive TSH assays and the thyrotropin-releasing hormone stimulation test offer alternative methods (see Chap. 15).