We describe the first reported case of symptomatic psychosis with the 'hallucination of soliloquy.' A 48[correction of 40]-year-old woman with Hashimoto disease exhibited of compulsive checking, mysophobia, and excessive hand washing. When these obsessive-compulsive symptoms diminished, she began to suffer from the 'hallucination of soliloquy', the automatic flow of meaningless words inside her mind. As the 'soliloquy' increased, her mood became unstable and she attempted suicide by analgesic ingestion. After this, she was admitted to the psychiatric ward of a general hospital. The administration of clomipramine (150 mg daily) decreased the 'soliloquy' symptoms, but they did not resolve. When hypothyroidism became available, thyroid hormone treatment (levothyroxine at 50 mg daily) was started. Four weeks later, her 'soliloquy' symptoms had almost resolved and after three months in a stable state, thyroid hormone treatment was stopped and her 'soliloquy' symptoms soon reappeared. After thyroid hormone treatment was resumed, her 'soliloquy' symptoms disappeared immediately. Typical auditory hallucinations and delusions of reference were not observed throughout the clinical course. We speculate that the symptoms were symptomatic psychosis induced by hypothyroidism secondary to Hashimoto disease, because the changes of her hallucinations were related to free T3 values and the symptoms disappeared soon after starting thyroid hormone treatment. The main features of this case were 'soliloquy' alternating with obsessive-compulsive symptoms, but her 'soliloquy' symptoms were thought to be autochthonous ideas rather than obsessive thoughts. Furthermore, the symptoms in this case were different from schizophrenia, since there was no disturbance of communication, and she had the sensation of both speaking and hearing her own voice. The psychopathology of this 'hallucination of soliloquy' may be related to the theory of 'vocalization of background thinking' (N. Nakayasu). Detailed observation of patients with symptomatic psychosis and a psychopathological description of their symptoms may help to contribute to the etiologic elucidation and treatment of psychosis.