Do thyroxine and thyroid-stimulating hormone levels reflect urinary iodine concentrations?

The toxicity of environmental chemicals such as nitrates, thiocynates, and perchlorates, some therapeutics, and dietary goitrogens can lower thyroidal iodine uptake and result in hypothyroidism and goiter. Iodine sufficiency, essential for normal thyroid hormone synthesis, is critical during gestation to assure that sufficient thyroxine (T-4) and iodine reach the developing fetus. Spot urinary iodide (UI) measurements are used globally to indicate and monitor iodine sufficiency of populations. In individuals, however, UI are not routinely measured; instead, normal serum thyroid-stimulating hormone (TSH) and T-4 concentrations serve as surrogate indicators of iodine sufficiency as well as thyroidal health. Our objective was to examine the relationship between UI concentrations and serum T-4 and TSH concentrations in individuals in an "iodine-sufficient population." Using a cross-sectional sample of the US population (n = 7628) from the National Health and Nutrition Examination Survey (NHANES 111; 1988-1994) database, we examined the relationship among UI, T4, and TSH in pregnant and nonpregnant women and in men (15-44 years). There was a lack of relationship between UI (or UI/Cr) concentrations and serum T-4 or TSH concentrations. Therefore, TSH and T-4 are not appropriate markers of UI concentrations in this population. Monitoring the status of iodine nutrition of individuals in the United States may be important because serum TSH and T-4 concentrations do not indicate low iodine status.