Results Radiation Effects of Theraneutic Doses of 131 I in the Thyroid in Clinical Subiects with Hyperthyroidis m Serial observations have been carried out on selected patients whom we treated with therapeutic doses of 13 ly for hyperthyroidism. and studied for many years thereafter. a minimal dose of i 31 The patients The important question I which will bring a hyperthyroid patient to without ultimately developing hypothyroidism. The other question ve been followed whether there is euthyroid state s whether an inadequate dose of 131; is more inclined to lead to neoplasm forma ion. Extensive pretreatment data was collected-on each patient sel special study. ted for this The uptake of 131 I by the thyroid, its disappearan e day by day from the gland, and the amount and the changing chemical form of radioa tivity in the circulation for several weeks, as well as the loss of isotope through the urine for 3 days, have been followed after the treatment dose. The results ob fained were ul- timately related to the changes that took place in the clinical pi tture and the degre of recovery from hyperthyroidism. These observations provided inf brmation from which the actual rad dose delivered to the thyroid could be determined. The therapeutic objective in the use of 1 a euthyroid state and no lower. 31 I was to bring the patient down to However, some patients developed hypothyroidism very rapidly in spite of the fact that the dose of 1 31 I per esti was thought to be appropriate to bring the patient slowly to the Other patients were relatively unaffected by what was calculated tive dose. ed gram of gland thyroid state. fo be a fully effec: The observer accuracy of the estimated weight of the glands was constant. checked by the same observer who was operating Similar surgical ci ses and saw the glands. Multiple doses were required in approximately one-fourth of these patients. In such cases, the serial studies following a subsequent treatmen | were Tepeated to see how the behavior of the 131, the second time commared with the first. When