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-sectional study of thyroid function. The mean urinary mercury concentration in the exposed workers was low compared with other studies of chloralkali workers: 5.9 nmol mmol-1 creatinine (range 1.1-16.8) vs 1.3 nmol mmol-1 creatinine (range 0.2-5.0) in the reference group. The median serum concentration of reverse triiodothyronine (rT3) was statistically significantly higher 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 difference between the exposed subjects and the referents was most pronounced in the highest exposed sub-groups. The free thyroxine (T4)/free T3 ratio was also 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 ml(-1) and range 6.0-34.6; P = 0.004). Exposed subjects with the lowest urinary iodine (<67.8 nmol mmol(-1) Cr) had higher serum concentrations of reverse T 3 and a higher free T4/free T3 ratio than the other subjects, suggesting that a low concentration of iodine in urine may be a risk factor for increased serum concentrations of reverse T3 and the free T4/free T3 ratio in subjects exposed occupationally to mercury vapour. The study could indicate a slight effect of low mercury vapour exposure on the function of the enzyme type I iodothyronine deiodinase, possibly modified by comparatively low urinary iodine concentrations.