Fluoride Exposure and Thyroid Function among Iodine Deficient Adults in Canada

Abstract

Background: Fluoride exposure has the potential to disrupt thyroid functioning, though adequate iodine intake may mitigate this effect. This study is the first population-based study to examine the impact of chronic low-level fluoride exposure on thyroid function, while considering iodine status.Objective: This study used weighted population-based data from Cycle 3 (2012-2013) of the Canadian Health Measures Survey (CMHS), a cross-sectional survey designed to collect health and wellness data and biological specimens on Canadians.Methods: The weighted sample represented 7,543,966 adults aged 18-79 who were not taking any thyroid-related medication. Urinary fluoride concentrations were measured in spot samples using an ion selective electrode and adjusted for specific gravity (UFSG) and creatinine (UFCR). Serum TSH levels provided a measure of thyroid function. Multivariable regression analyses examined the relationship between adjusted levels of urinary fluoride and TSH, controlling for covariates. We tested whether TSH levels were more strongly related to urinary fluoride levels in adults who were moderately-to-severely iodine deficient than in adults who were iodine sufficient, according to World Health Organization reference standardsResults: The mean (SD) age of the sample was 46.6 (15.8) years and approximately 17.8% fell in the moderately-to-severely iodine deficient range. Median UFSG and UFCR concentrations were 0.74 mg/L and 0.46 mg/g, respectively. Among iodine deficient adults, a 1 mg/L increase in UFSG was associated with a 0.36 mIU/L increase in TSH [95% CI: -0.03, 0.75; p = 0.03], and a 1 mg/g increase in UFCR was associated with a 0.61 mIU/L increase in TSH [95% CI: 0.22, 0.99; p < 0.01].Conclusion: Adults living in Canada who have moderate-to-severe iodine deficiencies and higher levels of urinary fluoride may be at an increased risk for underactive thyroid gland activity.