Effect of preparation method for radioactive iodine therapy on serum electrolytes

PURPOSE: Thyroid hormone withdrawal (THW) in preparation for radioactive iodine therapy (RIT) may lead to hyponatremia and hyperkalemia because hypothyroidism reduces the glomerular filtration rate. Using recombinant human thyrotropin (rhTSH) may avoid these changes; however, these two preparation methods have not been compared in the literature. The purpose of this study was to reveal whether THW and rhTSH as preparation methods for RIT affect serum electrolytes differently. We also evaluated clinical factors influencing the onset of hyponatremia and hyperkalemia during RIT. MATERIALS AND METHODS: From April 2005 to December 2020, we analyzed 278 patients with thyroid cancer who received RIT. The patients were classified into two groups based on the preparation method, and renal function and serum electrolytes were compared between the groups. We also evaluated clinical factors that may affect overt hyponatremia (serum sodium level < 134 mmol/L) and hyperkalemia (serum potassium level ≥ 5.0 mmol/L). RESULTS: Serum sodium and chloride levels in the THW group were significantly lower than those in the rhTSH group (p < 0.001 and p = 0.002, respectively). In contrast, the serum potassium level in the THW group was significantly higher than that in the rhTSH group (p = 0.008). As for clinical factors that may influence hyponatremia, age and estimated glomerular filtration rate (eGFR) were significantly associated with serum sodium level in the univariate analysis (p = 0.033 and p = 0.006, respectively). In the multivariate analysis, only age was significantly associated with serum sodium level (p = 0.030). Regarding hyperkalemia, distant metastases, the preparation method and eGFR were significantly associated with the serum potassium level in the univariate analysis (p = 0.005, p = 0.005 and p = 0.001, respectively). In the multivariate analysis, only eGFR was significantly associated with hyperkalemia (p = 0.019). CONCLUSION: THW and rhTSH affect serum sodium and potassium levels differently. Renal function may be risk factors for hyperkalemia, whereas older age may be a risk factor for hyponatremia.