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

Abstract

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.