Impact of lead exposure on pituitary-thyroid axis in humans

Thyroid function tests (serum levels of thyroxine-T4, triiodothyronine-T3 and thyroid stimulating hormone-TSH) were performed in fifty-eight men (mean age: 31.7 +/- 10.6 years; mean duration of lead exposure: 156.9 +/- 122.7 months). These subjects were exposed to lead either as petrol pump workers or automobile mechanics. The mean whole blood lead (Pb-B) levels were 2.49 +/- 0.45 mu mole/l (51.90 +/- 9.40 mu g/dl) in the lead exposed workers and were approximately 5 times higher than in the control (n = 35) subjects. No significant alteration was seen in their mean T3 and T4 levels as compared with the controls. Interestingly, T3 was significantly lower with the longer (210 months) exposure time in comparison with the group having shorter (29 months) exposure duration. The mean TSH levels were significantly (p < 0.01) higher in workers exposed in comparison with the control group. This rise in TSH was independent of exposure time, but it was definitely associated with the Pb-B levels. The increase being more pronounced with mean Pb-B levels of 2.66 +/- 0.2 mu mole/l (55.4 +/- 4.25 mu g/dl) when compared with the group having mean levels of 1.51 +/- 0.30 mu mole/l (31.5 +/- 6.20 mu g/dl). The rise is TSH associated with Pb-B levels was only statistical valid, however, the levels fall within the normal laboratory range. We thus conclude that the Pb-B levels of greater than or equal to 2.4 mu mole/l (50 mu g/dl) could enhance the pituitary release of TSH without having any significant alterations in the circulating levels of T3 and T4.