Risk factors for hyperkalemia in acutely hypothyroid patients undergoing thyroxine withdrawal before radioiodine therapy

Background Hyponatremia is a well known electrolyte abnormality seen in hypothyroidism. In contrast it is not well known if hypothyroidism causes hyperkalemia. Methods We studied retrospectively 108 Japanese patients with thyroid carcinoma undergone radioiodine therapy by the standard protocol of levothyroxine withdrawal at Nagasaki University Hospital since 2009 to 2013. Blood samples analyzed were at euthyroid status just before levothyroxine withdrawal and at hypothyroid status a week prior to take I-131. Frequency of hyperkalemia (K ≥ 5 mEq/L) and its associations with patient's age, serum levels of sodium (Na), creatinine (Cr), free T4 and TSH were studied. Results Five of 108 (4.6 %) patients developed hyperkalemia. Mean level of serum K (mEq/L) in acute hypothyroidism was significantly higher than those at euthyroid status (4.23 ± 0.50 vs. 4.09 ± 0.34, p < 0.001). At acute hypothyroid status, serum K values significantly correlated with age (r = 0.27, p = 0.004), serum levels of Na (r = -0.38, p < 0.001) and Cr (r = 0.26, p = 0.007), and estimated glomerular filtration rate (r = -0.34, p < 0.001), but did not correlated with free T4 (r = -0.004, p = 0.97) and TSH (r = -0.06, p = 0.56). Although the serum K levels in hypothyroid status (Hypo-K) showed a weak correlation with the serum K levels in euthyroid status (Eu-K) (r = 0.29, p = 0.003), the Hypo-K showed a strong correlation with the remainder after subtracting Eu-K from Hypo-K, which we defined as ΔK (r = 0.77, p < 0.001). Thus, ΔK seems to be a stronger contributing factor to the development of hyperkalemia in hypothyroid patients undergoing radioiodine therapy. Elevation of serum K > 0.5 mEq/L in hypothyroidism (ΔK > 0.5 mEq/L) was associated with patient's age over 60 years (OR = 4.66, p = 0.026) and also associated with the use of angiotensin-II receptor blocker or angiotensin-converting enzyme inhibitor (OR = 3.53, p = 0.033) among patient's age over 60 years in multivariate analysis. Conclusions Acutely hypothyroid patients undergoing withdrawal of a thyroxine replacement for radioiodine therapy may develop symptomatic hyperkalemia. Patients over 60 years old and/or under medical treatment with renin-angiotensin-aldosterone inhibitors in particular may have a greater risk of developing hyperkalemia, even if their serum K level is not high before levothyroxine withdrawal.