A 71-year-old man had severe hypothyroidism, chronic autoimmune thyroiditis, and bacteremia due to Edwardsiella tarda. Review of the literature identified the hypothesis that E tarda infections may occur more frequently in immunocompromised patients. Previous animal studies have shown decreases in lymphocyte function during hypothyroidism, with return of normal lymphocyte function during euthyroid states. Therefore, lymphocyte transformation studies were obtained, demonstrating severe decreases in our patient's lymphocyte function. Except for chronic autoimmune thyroiditis, other immune system abnormalities were excluded. Serial lymphocyte transformation studies showed gradual improvement in lymphocyte function during gradual return to euthyroid state. Possible pathophysiologic mechanisms for these findings will be reviewed.