Hypothyroidism and hyponatremia: data from a series of patients with iatrogenic acute hypothyroidism undergoing radioactive iodine therapy after total thyroidectomy for thyroid cancer.

PURPOSE: The aim of the present study was to evaluate the role of hypothyroidism as a cause of hyponatremia in a clinical model of iatrogenic acute hypothyroidism due to thyroid hormone withdrawal prior to ablative radioactive iodine (RAI) therapy after total thyroidectomy.

METHODS: The study group consisted of 101 differentiated thyroid cancer (DTC) patients (77 women and 24 men). Plasma concentration of thyroid-stimulating hormone ([TSH]) and sodium ([Na+ ]) was evaluated before total thyroidectomy (pre[TSH] and pre[Na+ ]) and on the day of RAI therapy (post[TSH] and post[Na+ ]).

RESULTS: The frequency of hypothyroidism-associated hyponatremia was 4 % (4/101). Pre[Na+ ] was significantly higher than post[Na+ ] (140.7 ± 1.6 vs 138.7 ± 2.3 mEq/L, p = 0.012). Moreover, a linear correlation was identified between pre[Na+ ] and post[Na+ ].

CONCLUSIONS: Iatrogenic acute hypothyroidism-related hyponatremia is uncommon. However, because of the significant reduction of [Na+ ] in the transition from euthyroidism to iatrogenic hypothyroidism, the value of pre[Na+ ] should be viewed as a parameter to be considered. Since it acts as an independent risk factor for the development of hyponatremia, patients with a pre[Na+ ] close to the lower limit of normal range may deserve a closer monitoring of [Na+ ].