The term postpartum psychosis refers to a group of severe and heterogeneous disorders with psychotic symptoms that occur most frequently in the context of a mood disorder during the postpartum period. We report a case of 'postpartum psychosis' possibly associated with postpartum thyroiditis in a 29 year-old woman. The appearance of psychotic symptoms was chronologically related to the onset of postpartum thyroiditis and resolution of psychosis synchronized with the achievement of biochemical euthyroidism. The patient had typical symptoms of 'classic postpartum psychosis' (a historical term not included in DSM-IV, but used frequently by many physicians to describe diagnostic and therapeutic challenges posed by puerperal psychoses). Three months postpartum, the patient began to believe that she was pregnant with the Christ child, although she was not pregnant. Her delusions resolved around the 'pregnancy' and harm to her 'unborn' child. She also believed that her child (Jesus) was going to be killed. Other key symptoms included hallucinations, mixed mood symptoms, agitation and transient disorientation. Her DSM-IV diagnosis on admission was major depression with psychotic features and her discharge diagnosis (most likely diagnosis) was psychotic disorder due to thyrotoxicosis caused by postpartum thyroiditis. The differential diagnosis of co-occurring psychosis and postpartum thyroiditis can be examined relative to four possibilities: (1) psychosis due to thyrotoxicosis caused by postpartum thyroiditis; (2) a coincidence (no association between psychosis and postpartum thyroiditis); (3) precipitation of psychotic symptoms and disorientation related to a postpartum thyroiditis in a woman with a pre-existing mood disorder; or (4) both psychosis and thyroiditis caused by a pre-existing defect in autoimmunity. The authors stress the importance of early diagnosis and prompt treatment of postpartum psychosis. They discuss the indications for thyroid screening in postpartum psychoses. Further research is needed to clarify the nosology and mechanisms of severe postpartum disorders and to elucidate treatment-relevant and etiologically-distinct subsets of postpartum psychosis.