Preparation for radioactive iodine administration in differentiated thyroid cancer patients

OBJECTIVE: Because in recent years the practice of TSH suppression has changed, and thyroxine doses have been reduced significantly in the treatment of patients with low-risk differentiated thyroid cancer, the goal of this study was to determine the time needed to attain a target TSH level (of 30 mIU/l) following levothyroxine withdrawal in patients treated with thyroxine according to current guidelines, in anticipation of radioactive iodine (RAI) administration. DESIGN: Observational study. PATIENTS: Thirteen consecutive patients with differentiated thyroid cancer on suppressive doses of levothyroxine planned for RAI administration. Five of the patients received cholestyramine in an attempt to facilitate TSH recovery. MEASUREMENTS: Serum TSH, free-T3 and free-T4, at 3 - 4-day intervals. RESULTS: In 13 patients on suppressive doses of thyroxine, on 15 separate occasions, baseline TSH levels were between 0.01 and 0.4 mIU/l. The mean interval required to reach the target TSH concentration of at least 30 mIU/l was 17 days (95% CI 15-19; range 11-28 days). Cholestyramine had no effect on the rate of TSH recovery. Once TSH concentration became detectable, it increased exponentially; and once it reached the upper limit of normal, it rarely took more than 10 days to attain target level. CONCLUSIONS: Attaining target TSH level before radioactive iodine administration requires a considerably shorter time than is currently recommended. Reducing preparation time might improve patients' acceptance of the procedure.