Hyponatremia is the most common electrolyte disorder in hospitalized patients, especially in elderly patients, in which morbidity varies widely in severity. A 64-year-old Thai woman with papillary thyroid cancer who developed hypothyroid state after thyroid hormone withdrawal for preparation of 1-131 treatment, had severe hyponatremia within the day of 1-131 administration. It is possible that the combination of old age, hypothyroidism, severe nausea and vomiting, and inappropriate secretion of antidiuretic hormone (SIADH) may all have precipitated the severe hyponatremia in the presented case.