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Preparation for radioactive iodine administration in differentiated thyroid cancer patients
1Endocrine Unit, Department of Internal Medicine C, Soroka University Medical Center of Clalit Health Services, PO Box 151, Beer-Sheva 84101, Israel. liel@bgumail.bgu.ac.il
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.