Thirty men occupationally exposed to lead have been examined. The selection of subjects was based on increased lead poisoning indices as well as on the data proving there had been no thyroid disease incidence noted prior to the lead poisoning. Each of the subjects underwent determination of T4 concentration simultaneously with the measurement of three basic indicators of lead poisoning: ALA in urine and ZPP and Pb in blood. Three T4 concentration ranges have been obtained therefore all the cases have been divided into three groups: I increased T4 concentration: II normal T4 concentration; III decreased T4 concentration. In each group T4 concentrations have been compared with the poisoning indicators. The highest poisoning indicators have been found in group II exhibiting normal T4 concentrations (43.33% of cases). It differed significantly from group III (50.0% of cases) exhibiting decreased T4 concentrations. The determination values in group I (increased T4 concentrations) have been excluded from statistical calculations, as there were too few cases (6.67%). The discrepancy of T4 results, their pathological picture at lower values of poisoning indices evidence that the establishment of Pb effects upon thyroid function would call for the determination of a number of additional factors possibly affecting the mechanism handling this phenomenon.