Factors Affecting Endogenous Thyroid-stimulating Hormone Rise after Thyroxine Withdrawal in Preparation for I-131 Whole-body Scan for Patients with Differentiated Thyroid Carcinoma
SM Chow, TS Choy, CG Morris, CK Kwan, PCM Poon, CC Shek, WM Mendenhall, SCK Law
Hong Kong J Radiol 2007;10:3-8
Objective: To assess the factors affecting thyroid-stimulating hormone level after thyroxine withdrawal in preparation for radioactive iodine whole-body scanning for patients with differentiated thyroid carcinoma.
Patients and Methods: This was a single-institute prospective study of 142 patients with differentiated thyroid carcinoma scheduled for whole-body scanning. All patients were athyreotic after thyroidectomy and radio-active iodine treatment. Serial serum free thyroxine and thyroid-stimulating hormone levels were checked at weeks 0, 2, 3, and 4.
Results: The mean serum thyroid-stimulating hormone level increased gradually after thyroxine withdrawal from 3.7 mIU/L (SD, 9.3 mIU/L) at week 0 to 39.7 mIU/L (SD, 38.0 mIU/L) at week 2, 93.0 mIU/L (SD, 56.0 mIU/L) at week 3, and 146.2 mIU/L (SD, 72.7 mIU/L) at week 4. The increase was greater in the last 2 weeks. A thyroid-stimulating hormone level of 30 mIU/L was reached in 92.9% of patients at 3 weeks and the mean time to attain this level was 18 days. Multivariate analysis showed that factors affecting the rise of thyroid-stimulating hormone were duration of thyroxine withdrawal (p < 0.0001), age (p = 0.04), and degree of thyroid-stimulating hormone suppression before thyroxine withdrawal (p < 0.0001).
Conclusions: To improve the acceptance of whole-body scanning by patients, it is feasible to change the protocol of thyroxine withdrawal prior to whole-body scanning from 4 to 6 weeks to 3 weeks. By that time, 92.9% of patients may attain a thyroid-stimulating hormone level of >30 mIU/L.