Excessive iodine induces thyroid dysfunction. However, the effect of excessive iodine exposure on maternal-fetal thyroid hormone metabolism and on the expression of genes involved in differentiation, growth and development is poorly understood. Since a thyroid hormone receptor response element was found in the Hoxc8 promoter region, Hoxc8 expression possibly regulated by excessive iodine exposure was firstly investigated. In the present study, Balb/C mice were given different doses of iodine in the form of potassium iodate (KIO3) at the levels of 0 (sterile water), 1.5, 3.0, 6.0, 12.0 and 24.0 microg/ml in drinking water for 4 months, then were mated. On 12.5 d postcoitum, placental type 2 and type 3 deiodinase activities and fetal Hoxc8 expression were determined. The results showed that excessive iodine exposure above 1.5 microg/ml resulted in an increase of total thyroxine and a decrease of total triiodothyronine in the serum of maternal mice, which was mainly associated with the inhibition of type 1 deiodinase activity in liver and kidney. Placental type 2 deiodinase activity decreased, showing an inverse relationship with maternal thyroxine level. Hoxc8 mRNA and protein expression at 12.5 d postcoitum embryos were down regulated. Because Hoxc8 plays an important role in normal skeletal development, this finding provides a possible explanation for the skeletal malformation induced by excessive iodine exposure and also provides a new clue to study the relationship between iodine or thyroid hormones and Hox gene expression pattern.