Low iodine diet with adequate amount of salt does not cause severe hyponatremia in patients undergoing rh-TSH aided remnant tissue ablation for thyroid cancer

Objectives Severe hyponatremia has been reported as a significant side-effect of a low iodine diet (LID) for radioiodine therapy for thyroid cancer. However, hyponatremia might have been combined effects of withdrawal of l-thyroxin and restriction of iodized salt rather than performance of LID. Thus, we investigated whether practice of LID decreases serum sodium (Na) or daily salt intake (SI) in patients who are to undergo remnant tissue ablation (RTA) for papillary thyroid cancer (PCA).

Methods 2 patients (M/F;17/25, age:27-77 yrs.) with post-operative PCA were included. The patients went on a one-week LID combined with diuretics as a preparation for rhTSH-aided RTA with low dose (1.11GBq) I-131. We developed more than 160 recipes that was designed to restrict dietary iodine intake less than100μg/day but not to restrict use of non-iodized salt and a handbook to help patients go on LID. The average nutrition facts of the meals per day were; 54.1±8.0 μg in iodine, 1.863±40 kcal in energy, 63.2±4.2 g in protein, 45.0±5.2 g in fat. and 6.1±0.4g in salt. After instruction a by a dietitian, LID was carried out from 7 days before to 2 days after administration of I-131, LID was combined with 40mg of oral furosemide from 7 days before to 1 day before administration of I-131. A spot urine sample was collected before start of LID and on the day of I-131 administration to determine urinary iodine concentration normalized by urinary creatinine (UIC; μg/gCRE) in all pts. Also, Na levels in the urine was determined in 24 pts to estimate daily sodium intake (SI:g/day) according to previous reports. Additionally, serum levels of Na(normal range:135-145mEq/l) was determined before start of LID and on the day of I-131 administration in all.

Results The average value of UIC, SI and Na after LID were about 11.5% (69.7 vs. 604.5, p<0.005), 55.0% (4.3 vs. 8.5, p<0.001) and 99.9% (140.1 vs.140.7, ns) of the baseline value, respectively. Although average amount of SI was significantly decreased after LID, it remained above the required minimal daily amount of 1.5g in all. Marginal hyponatremia was observed in only one (0.4%) of the pts. In the remaining 41 pts, Na level remained in normal range. There was a moderate correlation between UIC and SI after a LID (r=0,612, p=0.04) . In contrast, correlation between Na and SI was not significant (r=-0.32 p=0,335).

Conclusions Practice of LID may be associated with decrease in salt intake. However, it is unlikely that a one-week LID induces severe hyponatremia in patients undergoing rh-TSH aided remnant tissue ablation even if the procedure is combined with oral diuretics. Thoughtful use of non-iodized salt may be essential in prevention of hyponatremia.