Thyroid health status of ammonium perchlorate workers: A cross-sectional occupational health study

Since pharmaceutical exposures to perchlorate are known to suppress thyroid function in patients with hyperthyroidism, a study of employees at a perch lorate manufacturing plant was conducted to assess whether occupational exp osure to perchlorate suppresses thyroid function. Exposure to perchlorate w as assessed by measurement of ambient air concentrations of total and respi rable perchlorate particles, and systemic absorption was assessed by measur ement of urinary perchlorate excretion. Airborne exposures ranged from 0.00 4 to 167 mg total particulate perchlorate per day. Urinary perchlorate meas urements demonstrated that exposure to the airborne particulate perchlorate resulted in systemic absorption. Workers were grouped into four exposure c ategories with mean absorbed perchlorate dosages of 1, 4, 11 and 34 mg perc hlorate per day. Thyroid function was assessed by measurement of serum thyr oid-stimulating hormone, free thyroxine index, thyroxine, triiodothyronine, thyroid hormone binding ratio, thyroid peroxidase antibodies, and by clini cal examination. No differences in thyroid-function parameters were found b etween the four groups of workers across approximately three orders of magn itude of exposure end of dose. Thus human thyroid function was not affected by these levels of absorbed perchlorate. In addition, no clinical evidence of thyroid abnormalities was found in any exposure group. The blood-cell c ounts were normal in all groups, indicating no evidence of hematotoxicity i n this exposure range. The absence of evidence of an effect on thyroid func tion of blood cells from occupational airborne perchlorate exposure at a me an absorption of 34 mg/day demonstrates a no-observed-adverse-effect-level (NOAEL) that can assist in the evaluation of human health risks from enviro nmental perchlorate contamination.