Acute severe hypothyroidism is not associated with hyponatremia even with increased water intake: a prospective study in thyroid cancer patients

BACKGROUND Hypothyroidism, commonly induced in preparation for radioiodine treatment of differentiated thyroid cancer, is a text-book cause for hyponatremia. Nausea, stress, and increased fluid intake associated with the treatment are expected to exacerbate hyponatremia. METHODS We prospectively studied 212 (80% females) consecutive thyroid cancer patients for the incidence of hypothyroidism-induced hyponatremia and associated risk factors. RESULTS Mean(SD) age was 39.7(14.1) year, creatinine 82.0(20.8) μmol/l, TSH 141.6(92.0) mU/l, pre- and post-isolation sodium 139.5(2.3) and 137.8(3.0) mEq/l, respectively, and estimated fluid intake during isolation 9.7(6.2) L. 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. 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). 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). Pre-post-isolation drop in sodium concentration was more in females (mean difference 1.21, p = 0.02). Compared to eunatremic patients, hyponatremic patients were more likely to have pre-isolation hyponatremia (9% vs. 0.5%, p = 0.03), elevated creatinine concentration (36% vs. 13%, p = 0.008), and to be on diuretics (23% vs. 1%, p = 0.0001). CONCLUSIONS 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. Uncomplicated acute severe hypothyroidism didn't cause clinically-important hyponatremia/SIADH in this cohort of patients.