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Objective: To investigate the effects of pyrethroids on the concentrations of thyroid hormone in rat brain., Method: Permethrin (PM) and deltamethrin (DM) were administered to the rats with daily doses of 100, 200 and 400 mg.kg-1.d-1 and 6.25, 12.5 and 25 mg.kg-1.d-1, respectivelly for 15 days. Serum and brain tissue determinations of thyroxin (T4) and triiodothyronine (T3) were performed by radioimmunoassay (RIA)., Results: PM induced a dose dependent decrease in the serum levels of T4, T3, fT4 and fT3 and an increase in the serum TSH levels, whereas DM was only able to induce a dose dependent decrease in the serum levels of T4. PM treatment reduced both the levels of T4 and T3 in homogenates of the cerebral cortex and hippocampus respectively, whereas the highest dose of DM decreased only the cerebral cortex levels of T4. The effects of subchronic treatment with PM and DM on the concentrations of T3 were further investigated in the subcellular fractions, namely nuclei, mitochondria, myelin and synaptosomes of the cerebral cortex and hippocampus. PM treatment induced a decrease in the nuclear and synaptosomal concentrations of T3 of either the cerebral cortex or hippocampus, whereas DM reduced the levels of T3 especially in the mitochondria of the cortex and hippocampus., Conclusions: Treatment with pyrethroids subchronically to the rats would affect the serum and brain tissue levels of T4 and T3. These results indicate that the pyrethroids-induced neurotoxicity may involve at least in part an impairment of the physiological action of T3 at its subcellular targets.