Urine iodine measurements, creatinine adjustment, and thyroid deficiency in an adult United States population.

PURPOSE: The purpose of the study was to explore the relationships between urine iodine, creatinine, serum TSH, and total T(4) in a diverse population of U.S. adults with the aim of determining whether low urine iodine is associated with thyroid deficiency.
METHODS: Using the Health and Nutrition Surveys III data set, we examined median TSH and total T(4) values according to deciles of urine iodine (with and without creatinine correction). Stepwise regression analysis was used to further explore these relationships in the context of possible confounding variables. Exclusion criteria included age less than 21 yr, pregnancy, and the presence of thyroid antibodies.
RESULTS: Among the 5963 men and 5722 women, median urine iodine concentrations do not vary with increasing age, whereas median creatinine levels decrease (P < 0.001). Urine iodine and creatinine concentrations are lower among women (P < 0.001). TSH increases with age (P < 0.001), whereas total T(4) decreases (P < 0.001). Neither TSH nor total T(4) median values are associated with urine iodine. If the urine iodine to creatinine ratio is used instead, an extensive redistribution of study subjects occurs that results in an apparent positive relationship between this ratio and TSH measurements. A multivariate regression analysis that accounts for age, body mass index, race, creatinine, iodine, estrogen use, smoking, and gender reveals only a weak association between levels of urine iodine and markers of thyroid function.
CONCLUSIONS: Our analyses indicate that the U.S. nonpregnant adult population is iodine sufficient.