Long-Term Changes in Thyroidal Clearance Rate After Radioiodine Therapy

Thyroidal clearance rate measurements were made in 55 patients from 1 to 6 yr after receiving 131iodine therapy for thyrotoxicosis or nontoxic goiter. A regression of clearance rate on years after therapy was calculated for each patient. The slope of this regression line was taken as a measure of the long-term effect of therapy. The intercept of the line at zero yr (i.e., the time when the last therapy dose was administered) was used as a measure of the initial effect of therapy. The initial effect was found to correlate with estimated gram rads and millicuries retained at the first therapy dose. There was no correlation with goiter type. The longterm effect was correlated with me retained at the first dose. When this rate of fall of clearance rate was compared with the clearance rates obtained after therapy, a fall of 25% per annum was observed irrespective of thyroidal status before therapy. This was a 30-fold increase over the natural fall due to aging.