Elevated serum creatinine level in thyroid cancer patients undergoing withdrawal of throxine therapy for radioiodine scan/treatment

Chronic hypothyroidism is known to cause a significant reversible decrease in glomerular filtration rate [GFR], However, the effect on GFR of acute hypothyroidism, routinely induced in thyroid cancer patients in preparation for radioiodine scan/treatment, is not known. We studied the prevalence of abnormal serum creatinine level and the degree of its increase in hypothyroid patients with thyroid cancer four weeks after the withdrawal of thyroxine therapy. Creatinine level was measured in 116 patients on 191 hypothyroid episodes and in 56/116 and 18/116 patients while euthyroid or mildly hyperthyroid respectively. Abnormal creatinine level was significantly more prevalent in the hypothyroid state [34.5% vs 4% in the euthyroid or mildly hyperthyroid states] and significantly more common in males [50% vs 29% in females], in patients 150 mU/L [55% vs 30% with TSH 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 levels obtained in the euthyroid or mild hyperthyroid states, creatinine levels increased in the hypothyroid state on average 32% [23micro M/L, P=0.0001] with 24% of patients having >/= 50% increase. Elevated serum creatinine levels are rather common in thyroid cancer patienis undergoing temporary withdrawal of thyroxine treatment and more so in males, younger patients or in association with higher TSH levels. Since the clearance of iodine is linearly related to GFR, our study suggests that in the setting of hypothyroidism, the bioavailability of a given dose of radioiodine may have significant individual variation

Influence of age and sex in modulating TSH level in primary hypothyroidism

Previous studies addressing the interaction of age and sex with the function of the hypothalamic-pituitary-thyrotrophs axis yielded conflicting results, due in part to inability to control for the effect of variable free thyroid hormone levels. We studied the effect of age and sex on TSH levels in patients with severe primary hypothyroidism who have essentially undetectable plasma thyroid hormone levels. The TSH levels were measured in 116 thyroid cancer patients four weeks after the withdrawal of thyroxine therapy in preparation for radioiodine scan/treatment. All patients had a TSH >/= 30 mU/L [normal = 0.2-5] and a free T[4] <6 pmol/L [normal = 10-25]. Thirty males and 86 females with a mean age [ +/- SD] of 40 +/- 16 [range 6-89 years] were studied on up to four hypothyroid episodes, with a total of 191 episodes. The TSH level during the first hypothyroid episode correlated significantly with the TSH level during subsequent episodes [first episode versus second episode, r = 0.7, P - 0.0001; first versus third episode, r = 0.6, P - 0.03]. There was a significant negative correlation between age and TSH level [r = -0.24, P - 0.0009] that persisted when only the first hypothyroid episode was considered [r = -0.23, P = 0.01], or when only males [r = -0.32, P = 0.02] or only females [r = -0.23, P = 0.005] were considered. Means of TSH levels in males and females were not significantly different [130 versus 114 mU/L, respectively; P = 0.28]. We conclude that age but not sex may modulate the sensitivity/responsiveness of the hypothalamic-pituitary thyrotroph axis to primary hypothyroidism