Does thyroid disruption contribute to the developmental neurotoxicity of chlorpyrifos?

Although organophosphate pesticides are not usually characterized as "endocrine disruptors," recent work points to potential, long-term reductions of circulating thyroid hormones after developmental exposures to chlorpyrifos that are devoid of observable toxicity. We administered chlorpyrifos to developing rats on gestational days 17-20 or postnatal days 1-4, regimens that produce distinctly different, sex-selective effects on neurobehavioral performance. The prenatal regimen produced a small, but statistically significant reduction in brain thyroxine levels from juvenile stages through adulthood; in contrast, postnatal exposure produced a transient elevation in young adulthood. However, in neither case did we observe the sex-selectivity noted earlier for neurobehavioral outcomes of these specific treatment regimens, or as reported earlier for effects on serum T4 in developing mice. Thus, although chlorpyrifos has the potential to disrupt thyroid status sufficiently to alter brain thyroid hormone levels, the effect is small, and any potential contribution to neurobehavioral abnormalities remains to be proven.