Malin AJ, Riddell J, McCague H, Till C - "Fluoride exposure and thyroid function among adults living in Canada: Effect
modification by iodine status" Environ Int 121(Pt 1):667-674 (2018)
BACKGROUND: Fluoride exposure has the potential to disrupt thyroid functioning,
though adequate iodine intake may mitigate this effect. This is the first
population-based study to examine the impact of chronic low-level fluoride
exposure on thyroid function, while considering iodine status. The objective of
this study was to determine whether urinary iodine status modifies the effect of
fluoride exposure on thyroid stimulating hormone (TSH) levels.
METHODS: This cross-sectional study utilized weighted population-based data from
Cycle 3 (2012-2013) of the Canadian Health Measures Survey (CHMS). Information
was collected via a home interview and a visit to a mobile examination centre.
The weighted sample represented 6,914,124 adults in Canada 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). Serum TSH levels provided a measure of thyroid function.
Multivariable regression analyses examined the relationship between UFSG and TSH,
controlling for covariates.
RESULTS: Approximately 17.8% of participants fell in the moderately-to-severely
iodine deficient range. The mean (SD) age of the sample was 46.5 (15.6) years and
the median UFSG concentration was 0.74 mg/L. Among iodine deficient adults, a
1 mg/L increase in UFSG was associated with a 0.35 mIU/L increase in TSH [95% CI:
0.06, 0.64; p = 0.01, one-tailed].
CONCLUSIONS: 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.
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