FIGURE 3. Time–activity curves from patient 1 after diagnostic ¹³¹I. Radioactivity declines in first 2 studies but then rises and plateaus as radioiodide is converted into secreted radiothyroxine. Initial declines in radioactivity are slower before treatments 2, 4, and 5 than before treatment 1 because radioiodide is being sequestered less rapidly by thyroid tumors. Rx = treatment.