Impact of Smoking and Thiocyanate on Perchlorate and Thyroid Hormone Associations in the 2001–2002 National Health and Nutrition Examination Survey

BACKGROUND: Findings from a recent large study suggest that perchlorate at commonly occurring exposure concentrations may decrease thyroid hormone levels in some women. Decreases in thyroid hormone seen with perchlorate exposure could be even greater in people with concomitant exposure to agents such as thiocyanate that may affect the thyroid by mechanisms similar to those of perchlorate. OBJECTIVES AND METHODS: We used data from the National Health and Nutrition Examination Survey to assess the impact of smoking and thiocyanate on the relationship between urinary per-chlorate and serum thyroxine (T(4)) and thyroid-stimulating hormone (TSH). RESULTS: In women with urinary iodine levels < 100 μg/L, the association between the logarithm of perchlorate and decreased T(4) was greater in smokers [regression coefficient (β) = −1.66, p = 0.0005] than in nonsmokers (β = −0.54, p = 0.04). In subjects with high, medium, and low cotinine levels, these regression coefficients were −1.47 (p = 0.0002), −0.57 (p = 0.03), and −0.16 (p = 0.59). For high, medium, and low thiocyanate tertiles they were −1.67 (p = 0.0009), −0.68 (p = 0.09), and −0.49 (p = 0.11). Clear interactions between perchlorate and smoking were not seen with TSH or with T(4) in women with urinary iodine levels ≥ 100 μg/L or in men. CONCLUSIONS: These results suggest that thiocyanate in tobacco smoke and perchlorate interact in affecting thyroid function, and this effect can take place at commonly occurring perchlorate exposures. Agents other than tobacco smoke might cause similar interactions, and further research on these agents could help identify people who are particularly susceptible to perchlorate.