Effects of low mercury vapour exposure on the thyroid function in chloralkali workers

Forty-seven chloralkali workers exposed to mercury vapour for an average of 13.3 years were compared with 47 referents matched for age in a cross-sect ional study of thyroid function. The mean urinary mercury concentration in the exposed workers was low compared with other studies of chloralkali work ers: 5.9 nmol mmol(-1) creatinine (range 1.1-16.8) vs 1.3 nmol mmol(-1) cre atinine (range 0.2-5.0) in the reference group. The median serum concentrat ion of reverse triiodothyronine (rT(3)) was statistically significantly hig her in the exposed subjects compared with the referents (268 pmol l(-1) and range 161-422 vs 240 pmol l(-1) and range 129-352; P=0.009), The differenc e between the exposed subjects and the referents was most pronounced in the highest exposed subgroups. The free thyroxine (T-4)/free T-3 ratio was als o higher in the highest exposed subgroups compared with the referents. The median serum concentration of tumour necrosis factor alpha (TNF-alpha) was lower in the exposed subjects (7.3 pg ml(-1) and range 4.4-69.7 vs 8.0 pg m l(-1) and range 6.0-34.6; P = 0.004), Exposed subjects with the lowest urin ary iodine (<67.8 nmol mmol(-1) Cr) had higher serum concentrations of reve rse T-3 and a higher free T-4/free T-3 ratio than the other subjects, sugge sting that a low concentration of iodine in urine may be a risk factor for increased serum concentrations of reverse T-3 and the free T-4/free T-3 rat io in subjects exposed occupationally to mercury vapour. The study could in dicate a slight effect of low mercury vapour exposure on the function of th e enzyme type I iodothyronine deiodinase, possibly modified by comparativel y low urinary iodine concentrations. Copyright (C) 2000 John Wiley & Sons, Ltd.