What makes practice of low iodine diet difficult in patients undergoing remanat tissued ablation for thyroid cancer?

Objectives Low iodine diet (LID) is an important preparation of radioiodine therapy for thyroid cancer. However, practice of stringent LID for longer duration could be both annoying and boring. The aim of this study was to determine the relationship between personal characteristics and outcome of LID in patients to undergo remnant tissue ablation for papillary thyroid cancer.

Methods This study included 40 patients with postsurgical papillary thyroid carcinoma who were to undergo rh-TSH aided remnant tissue ablation using 30mCi of I-131. Pathologic finding was either pT3 or pT4, or pN1a or pN1b in all. After counseling by a dietitian, LID was done from 7 days before to 3 days after administration of I-131. We evaluated relationship between clinical and social characteristics of patients vs. outcome of LID. Urinary iodine concentration normalized by urinary creatinine (UIC: μg/gCRE) was measured before start of LID (UIC1) and on the day of I-131 ablation (UIC2) using a spot urine sample. Based on UIC2, outcome of LID was defined as successful (UIC2 < 100) or unsuccessful (UIC2 蠅 100). As possible clinical factors affecting performance of LID, age, gender, BMI, estimated GFR, daily dose of l-thyroxin, and UIC1 were focused on. Similarly, residence, frequency of dinning out during the LID period, possession of professional job, habit of self-cooking, use of soup stock from fish or sea weed before LID, and previous experience of dietary counseling with a dietitian were focused on as life style-related characteristics. Radioiodine uptake in the remnant tissue (RU) was semi-quantitatively assessed using a gamma camera at 3 days after ingestion of I-131. The relationship between clinical or life stile related factors and successful rate of LID were determined.

Results Both the UIC2 and the UIC1 ranged widely from 19 to 1850 (average 151) and 25 to 7840 and (average 713) , respectively. 31 out of 40(78%) patients were assigned to successful LID while the remaining 9 (22%) was assigned to unsuccessful LID. There was significant difference in the average of I-131 uptake on while body scan between patients with successful LID and those with unsuccessful LID (10.1 vs.6.0, p60 yrs.) and UIC1 (>300) were significantly related to outcome of LID. The older patients and those with higher daily iodine intake before LID have higher risk of unsuccessful LID with an odd ratio of 11.1 (95%CI: 1.21‐101.7) and 12.7(95%CI: 1.21‐101.7), respectively.

Conclusions Practice of LID may be difficult in older patients or patients with higher UIC under routine diet. Since risk of failure of LID increases in such group, it is recommended LID plan for rh-TSH aided remnant tissue ablation should be applied on a patient-to-patient basis taking patients’ personal characteristics in to consideration