Depressed thyroid indexes associated with occupational exposure to inorganic lead

The finding of low values for serum thyroxine and estimated free thyroxine in seven of 12 workers referred because of elevated blood lead levels (greater than 40 mg/L) prompted further investigation. In a cross-sectional study of workers at a small foundry, both measurements were found to regress negatively with blood lead 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 are compatible with a central depression of the thyroid axis or an alteration in thyroxine metabolism or binding to proteins. Irrespective of mechanism, the association between low thyroid indexes and elevated lead levels merits attention because of the large number of workers exposed to lead and the similarities between the clinical features of adult lead poisoning and hypothyroidism.