Severe hyponatremia following radioactive iodine therapy in patients with differentiated thyroid cancer

Abstract

Background: Symptomatic hyponatremia in association with radioactive iodine (RAI) therapy for differentiated thyroid cancer has rarely been reported. Due to the increasing incidence of thyroid cancer worldwide, more patients are now receiving postoperative RAI therapy. We report two cases of life-threatening severe hyponatremia in association with RAI therapy.

Summary: Two elderly female patients who had bilateral thyroidectomies for their thyroid cancer underwent a low-iodine diet and levothyroxine withdrawal for two weeks prior to RAI therapy. Upon admission, the patients were given 130 mCi (4810 MBq) and 150 mCi (5550 MBq) of 131I respectively, and oral hydration (two to three liters of water daily) to increase the frequency of emptying the bladder of RAI. Both patients completed their RAI therapy without significant complications and were discharged from hospital. Two days after discharge, both patients were admitted to the emergency room with complaints of severe nausea and dizziness. Initial laboratory tests revealed that they were in a hypothyroid state and had severe hyponatremia with a serum sodium level of 108 mEq/L. The symptomatic hyponatremia responded to intravenous hypertonic saline infusion and thyroid hormone replacement, and the patients made a full recovery. The low-iodine diet and hypothyroid state with overzealous hydration in the setting of RAI therapy may have provoked severe hypotonic hyponatremia.

Conclusion: Hypothyroid patients after undergoing RAI therapy, especially the elderly, are at an increased risk for serious hyponatremia and should be monitored closely.