The possible role of viruses as the cause of some thyroid disorders was evaluated in three strains of mice neonatally infected with lymphocytic choriomeningitis virus. We report the first definitive evidence that viruses can persist in the thyroid gland, particularly thyroid epithelial cells in which thyroglobulin is synthesized. Concomitant with the infection of these cells was a significant reduction in the level of thyroglobulin mRNA and circulating thyroid hormones. Another virus that causes persistent infection but does not replicate in the thyroid gland failed to alter serum levels of thyroid hormones, indicating the thyroid dysfunction was not a generalized result of stress accompanying a persistent infection. This alteration in thyroid homeostasis during persistent infection with lymphocytic choriomeningitis virus is not caused by autoantibodies to the thyroid. Moreover, despite infection of the thyroid gland, neither necrosis nor inflammation occurs. Thyroid dysfunction was noted both when persistence was initiated at birth and in utero during congenital infection. These observations in an experimental model raise the issue that viruses may play a role in the pathogenesis of some thyroid disorders in man. (Endocrinology122: 567–575, 1988)