![]() ![]() Several cases of iatrogenic hyponatremia have been reported in this setting, suggesting that sodium concentration obtained before radioiodine treatment may not indicate the true incidence/degree of hyponatremia. The anxiety associated with isolation, radioiodine treatment, and fear of bad prognosis may further stimulate antidiuretic hormone secretion. Furthermore, radioiodine administration may cause nausea which is a very potent stimulus of antidiuretic hormone secretion. Low iodine diet involves salt restriction, and increased fluid intake is typically recommended to “flush out” radioiodine from the gastrointestinal and urinary systems factors that may further aggravate hypothyroidism-associated hyponatremia. In preparation for radioiodine therapy, thyroid hormone treatment is typically withheld and patients are instructed to follow low iodine diet. In the management of patients with differentiated thyroid cancer (papillary and follicular), surgery is the primary therapy and radioiodine is used for ablation of residual thyroid tissue as well as in the treatment of residual tumor and metastatic disease. The mechanisms of hyponatremia in chronic hypothyroidism are not well understood but may involve decreased water clearance and inappropriate concentrations of antidiuretic hormone, and may not apply to acute hypothyroidism. The low prevalence of hyponatremia in that retrospective study may have been due to the fact that sodium concentrations were only determined pre-isolation for radioiodine treatment. However, we have previously reported that in the setting of thyroid hormone therapy withdrawal in patients with differentiated thyroid cancer, only 3.9% of 128 patients had mild hyponatremia and none had severe hyponatremia. Hypothyroidism is a text-book cause of hyponatremia. Uncomplicated acute severe hypothyroidism didn’t cause clinically-important hyponatremia/SIADH in this cohort of patients. In the setting of acute severe hypothyroidism: 1) clinically-important hyponatremia is uncommon sodium concentration may not need to be monitored unless patients have impaired renal function or are on diuretics, 2) age and female gender are associated with lower sodium concentration. ![]() Compared to eunatremic patients, hyponatremic patients were more likely to have pre-isolation hyponatremia (9% vs. Pre-post-isolation drop in sodium concentration was more in females (mean difference 1.21, p = 0.02). ![]() ![]() There was significant correlation between post-isolation sodium concentration and age (r = −0.24, p < 0.0001) and creatinine concentration (r = −0.22, p = 0.001). There was no significant correlation between post-isolation sodium concentration and TSH concentration (r = 0.03, p = 0.69) or estimated fluid intake (r = 0.10, p =0.17). Mild hyponatremia (≥130 mEq/l) was present in 18 patients (8.5%) and moderate hyponatremia (≥120 mEq/l) in 4(1.9%), 3 of the latter had elevated creatinine concentration and 2 were on diuretics. We prospectively studied 212 (80% females) consecutive thyroid cancer patients for the incidence of hypothyroidism-induced hyponatremia and associated risk factors. Nausea, stress, and increased fluid intake associated with the treatment are expected to exacerbate hyponatremia. Hypothyroidism, commonly induced in preparation for radioiodine treatment of differentiated thyroid cancer, is a text-book cause for hyponatremia. ![]()
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