Thyroid Uptake of Liquid Versus Capsule 131 I Tracers in Hyperthyroid Patients Treated with Liquid 131 I

The amount of ¹³¹I used to treat hyperthyroid patients is based in part on the 24-hour thyroid uptake of a diagnostic amount of radioiodine (tracer). We compared the 24-hour uptake of an 131I tracer administered in liquid or capsule form to the 24-hour uptake of ¹³¹I therapy administered as liquid. Sixty-five hyperthyroid patients with Graves' disease were evaluated and subsequently treated with radioiodine. The liquid group (45 patients) received a liquid ¹³¹I tracer (1.85 MBq [0.05 mCi]) and the capsule group (20 patients) received a capsule ¹³¹I tracer (1.63 MBq [0.044 mCi]). Probe calibration factors were the same for the liquid and capsule ¹³¹I standards. All patients received therapeutic amounts of ¹³¹I [114.7-1106.3 MBq [3.1-29.9 mCi]) in liquid form. Therapy uptakes were obtained using the same collimated uptake probe modified with a calibrated lead shield to attenuate the high photon flux. The mean therapeutic uptake was the same for both groups (58%). The mean diagnostic uptake for the capsule group, however, was less than the mean diagnostic uptake for the liquid group (44% vs. 63%). The mean diagnostic uptake for the capsule group was significantly lower than the mean therapeutic uptake for this group (44% vs. 58%), whereas the mean diagnostic and therapeutic uptakes were similar for the group receiving a liquid tracer (63% vs. 58%). In conclusion, diagnostic uptakes performed with a liquid tracer more accurately predicted liquid therapy uptakes than diagnostic uptakes performed with a capsule tracer. This raises concern about the bioavailability of ¹³¹I in capsule form and has implications for determining the amount of ¹³¹I to administer for therapy. Patients whose ¹³¹I therapy was based on the uptake of a capsule tracer received a higher than intended amount of radiation to the thyroid gland.