Depressed thyroid indexes associated with occupational exposure to inorganic lead

Abstract

The finding of low values for serum thyroxine and estimated free thyroxine in7 of 12 workers referred because of elevated blood Pb levels (> 40 mg/l) prompted further investigation. In a cross-sectional study of workers at a small foundry, both measurements regressed negatively with blood Pb level. In 12 of 47 subjects, both indexes were in the hypothyroid range. Serum thyrotropin and triiodothyronine levels in patients and study subjects with low indexes were all normal. Physical examinations failed to demonstrate the classic features of hypothyroidism. These data were compatible with a central depression of the thyroid axis or an alteration in thyroxin metabolism or binding to proteins. Irrespective of mechanism, the association between low thyroid indexes and elevated Pb levels was important because of the large number of workers exposed to Pb and the similarities between the clincial features of adult Pb poisoning and hypothyroidism.