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The Effect of Perchlorate, Thiocyanate, and Nitrate on Thyroid Function in Workers Exposed to Perchlorate Long-Term

Section of Endocrinology, Diabetes, and Nutrition (L.E.B., X.H., S.P.) and Departments of Radiology (M.C.) and Laboratory Medicine (B.M.), Boston Medical Center, Boston, Massachusetts 02118; Consultants in Epidemiology and Occupational Health (S.H.L., M.B.K., A.E.), Washington, DC 20007; Environ (K.S.C.), Ruston, Louisiana 71270; and Health Management Division (J.P.G.), Kerr-McGee Shared Services LLC, Oklahoma City, Oklahoma 73125

Address all correspondence to: Lewis E. Braverman, Boston University School of Medicine, 88 East Newton Street, Evans Building, Room 201, Boston, Massachusetts 02118-2347. E-mail: Lewis.Braverman{at}bmc.org. Address reprint requests to: John P. Gibbs, M.D., P.O. Box 25861, Oklahoma City, Oklahoma 73125. E-mail: jpgibbs{at}kmg.com.

Perchlorate (ClO₄^–) and thiocyanate (SCN^–) are potent and nitrate (NO₃^–) a weak competitive inhibitor of the thyroid sodium-iodide symporter. To determine the effects of long-term, high ClO₄^– exposure on thyroid function, we conducted a study of 29 workers employed for at least 1.7 yr (50% over 5.9 yr) in an ammonium ClO₄^– production plant in Utah. Serum ClO₄^–, SCN^–, and NO₃^–; serum T₄, free T₄ index, total T₃, thyroglobulin (Tg), and TSH; 14-h thyroid radioactive iodine uptake (RAIU); and urine iodine (I) and ClO₄^– were assessed after 3 d off (Pre) and during the last of three 12-h night shifts in the plant (During) and in 12 volunteers (C) not working in the plant. Serum and urine ClO₄^– were not detected in C; urine ClO₄^– was not detected in 12 of 29 and was 272 µg/liter in 17 Pre workers; serum ClO₄^– was not detected in 27 of 29 Pre; and serum and urine ClO₄^– were markedly elevated during ClO₄^– exposure to 868 µg/liter and 43 mg/g creatinine, respectively. Serum SCN^– and NO₃^– concentrations were similar in all groups. Thyroid RAIUs were markedly decreased in During compared with Pre (13.5 vs. 21.5%; P < 0.01, paired t) and were associated with an increase in urine I excretion (230 vs. 148 µg I/g Cr; P = 0.02, paired t) but were similar to those in the C group (14.4%). Serum TSH and Tg concentrations were normal and similar in the three groups. Serum T₄ (8.3 vs. 7.7 µg/dl), free T₄ index (2.4 vs. 2.2), and total T₃ (147 vs. 134 ng/dl) were slightly but significantly increased in the During vs. Pre workers (P < 0.01, paired t). Thyroid volumes and patterns by ultrasound were similar in the 29 workers and 12 community volunteers. In conclusion, high ClO₄^– absorption during three nights work exposure decreased the 14-h thyroid RAIU by 38% in ClO₄^– production workers compared with the RAIU after 3 d off. However, serum TSH and Tg concentrations and thyroid volume by ultrasound were not affected by ClO₄^–, suggesting that long-term, intermittent, high exposure to ClO₄^– does not induce hypothyroidism or goiter in adults.

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