ELEVATED SERUM CREATININE LEVEL IN THYROID-CANCER PATIENTS UNDERGOINGWITHDRAWAL OF THYROXINE THERAPY FOR RADIOIODINE SCAN TREATMENT

Chronic hypothyroidism is known to cause a significant reversible decr ease in glomerular filtration rate (GFR). However, the effect on GFR o f acute hypothyroidism, routinely induced in thyroid cancer patients i n preparation for radioiodine scan/treatment, is not known. We studied the prevalence of abnormal serum creatinine level and the degree of i ts increase in hypothyroid patients with thyroid cancer four weeks aft er the withdrawal of thyroxine therapy. Creatinine level was measured in 116 patients on 191 hypothyroid episodes and in 56/116 and 18/116 p atients while euthyroid or mildly hyperthyroid respectively. Abnormal creatinine level was significantly more prevalent in the hypothyroid s tate (34.5% vs 4% in the euthyroid or mildly hyperthyroid states) and significantly more common in males (50% vs 29% in females), in patient s less than or equal to 31 years old (48% vs 26% in older patients) an d in patients with a TSH level >150 mU/L (55% vs 30% with TSH less tha n or equal to 150 mU/L). Analyzing data on females only or including a ll hypothyroid episodes did not significantly alter the results. Furth er, compared to patients with normal creatinine level, patients with a bnormal creatinine levels were significantly younger (in the whole gro up, mean age 35.1 vs 42.5 years; in the subgroup of patients with a TS H level >150 mU/L, 29.8 vs 41.4 years; in females, 28.3 vs 42.5 years) and there was a significant negative association between the presence of abnormal creatinine levels and different age groups. Compared to l evels obtained in the euthyroid or mild hyperthyroid states, creatinin e levels increased in the hypothyroid state on average 32% (23 mu M/L, P=0.0001) with 24% of patients having greater than or equal to 50% in crease. Elevated serum creatinine levels an rather common in thyroid c ancer patients undergoing temporary withdrawal of thyroxine treatment and more so in males, younger patients or in association with higher T SH levels. Since the clearance of iodine is linearly related to GFR, o ur study suggests that in the setting of hypothyroidism, the bioavaila bility of a given dose of radioiodine may have significant individual variation.