The Efficacy of Simplified One-Week Low Iodine Diet and One-week Thyroid Hormone withdrawal Protocol for Preparation of Radioiodine rhTSH Scanning in Korea Patients.

Objectives The patients with rhTSH do not suffer from hypothyroidism symptoms because administering rhTSH in the euthyroid state. Standardized rhTSH protocol has not been well established but 2 weeks of low iodine diet (LID) is commonly recommended prior to rhTSH diagnostic whole body scan (DxWBS). LID for 2 weeks is problematic in Korea because the dietary iodine intake is higher than other countries and seaweeds are the most important source of iodine. Moreover, LT4 and LT3 contains considerable amount of iodinie, this it can be an hindrance factor for LID success. Besides on thiese points, we combined 1 week simplified LID (seaweed restriction only) with 1 week thyroid hormone withdrawal for successful LID without intoloralbe hypothyroidism symptoms and discomfort in diet and applied it to the patients with preparing rhTSH DxWBS. We want to evaluate the efficacy of this compromise plan and compare it with conventional ( 2 weeks LID and withdrawal LT4 for 4 weeks and swithched to LT3 for 2 weeks followed by stopping LT3 for 2 weeks) protocol by using spot urine analysis.

Methods Total 139 papaillary thyroid cancer patients preparing for rhTSH DxWBS whe were received hign dose RAIT with conventional protocol 6 months prior to the DxWBS were enrolled. During the preparation of RAIT, morning spot urine specimens were obtained three times (a week after starting LID (LID1) and 2 weeks after LID protocal (LID2)) from each patient and the urine iodine excretion (UIE) and urine I/Cr level were checked to evaluate the efficacy of LID. For rhTSH DxWBS, they went on a compromised plan and morning spot urine was obtained prior to DxWBS and UIE and urine I/Cr level were also evaluated. The successful LID criteria was below 100μg/L for UIE and 100μg/g for urine I/Cr.

Results The mean UIE is 265μg/L(median 117.9μg/L) and the mean I/Cr level is 273.5μg/g (median 107.79μg/g) after new protocol. Our data reveal that failure to reach targeted LID. These are significantly high compared with LID1 and LID2. However, worth noting is that these levels are quiet low in the consideration of the average UIE is as high as 674μg/L in Korea.

Conclusions From our result, more than a week period is neccessary to acheive an optimal level of iodine restriction for rhTSH DxWBS with simplfied LID.