Background: Macrophages and polymorphonuclear cells (PMNs) play an important role in the first line of defense against bacteria by infiltrating the infected organ in order to clear the harmful pathogen. Our earlier studies showed that granulocytes express type 3 deiodinase (D3) when activated during a turpentine-induced abscess. We hypothesized that D3 expression by granulocytes may also occur during bacterial infection. Methods: In order to test this hypothesis, we used the following experimental infection models: peritonitis induced by Escherichia coli and acute pneumonia induced by Streptococcus pneumoniae. Results: E. coli-induced peritonitis was characterized by infiltration in the liver by inflammatory cells with abundant immunocytochemical D3 expression while no staining was present in hepatocytes of infected or control mice. Acute pneumonia induced by S. pneumoniae resulted in inflamed lungs characterized by numerous infiltrating granulocytes expressing D3 while no D3 staining was present in lung sections without an infiltrate. Serum thyroid hormones were negatively correlated to bacterial outgrowth in both lung and spleen, and thus to the severity of illness. Conclusion: Infiltrating granulocytes during acute bacterial infection express D3. Our work supports the hypothesis that D3 plays an important role during chemical and bacterial inflammation. Whether the resulting decreased local bioavailability of thyroid hormones or rather the increased local availability of iodide is an important element of the innate immune response remains to be studied.