Hypothyroidism is widely accepted as a cause of hyponatremia and hypercreatininemia. However, the prevalence and severity of hyponatremia and hypercreatininemia in hypothyroid patients without comorbid conditions have not been well documented. We retrospectively studied serum sodium and creatinine levels in thyroid-ablated patients with differentiated thyroid cancer off (no.=128) and on (no.=60) thyroid hormone therapy. In the hypothyroid state, mean(+/-SD) TSH, sodium, and creatinine levels were 130.3+/-104.8 mU/l, 139.3+/-2.7 mEq/l, and 89.4+/-20 mmol/l respectively. Twenty-four patients (18.8%) had creatinine levels above the age- and sex-adjusted normal range, whereas only five patients (3.9%) had sodium levels below 135 mEq/l. No patient had a sodium level less than 130 mEq/l. Compared to their euthyroid values, mean sodium and creatinine levels of the hypothyroid patients changed by -1.18 mEq/l (p=0.003) and 17.2 mmol/l (p<0.0001), respectively. There was significant correlation of TSH levels in the hypothyroid state with the changes from the euthyroid state to the hypothyroid state in creatinine levels (r=0.29, p=0.02) but not with the corresponding changes in sodium levels (r=0.06, p=0.6). In thirty-seven patients studied in two hypothyroid episodes, there was a significant correlation between a) TSH levels in hypothyroid state 1 and hypothyroid state 2 (r=0.56, p=0.0003), and b) the change in creatinine levels from the euthyroid state to hypothyroid state 1 and the corresponding change from the euthyroid state to hypothyroid state 2 (r=0.48, p=0.003). There was no significant correlation between the change in sodium levels from the euthyroid state to hypothyroid state 1 and the corresponding change from the euthyroid state to hypothyroid state 2 (r=0.32, p=0.05). We conclude that hyponatremia is very uncommon, whereas mild to moderate elevation in serum creatinine level is not uncommon in patients with short-term uncomplicated hypothyroidism.