Lyme disease can be seen as localized, disseminated, acute, or chronic and can mimic other, more serious diseases. Even though it is a multisystemic illness, very few spirochetes are present; yet, once established in the host, it can persist for years. The antibody response is slow and variable, and the spirochete is difficult to isolate from clinical specimens, even those obtained from the pathognomonic skin lesion. These variables, together with nonspecific symptoms, make the diagnosis of Lyme disease difficult. The author describes an unusual case of Lyme disease superimposed on severe primary hypothyroidism in which the thyroid disorder was so advanced and pronounced that the diagnosis of Borrelia infection could easily have been missed.