To the Editor: Interferon alpha (INF-α), which is antiviral, immunomodulatory, and antiproliferative, is currently used to treat carcinomas and chronic viral infections. Although it is established in the psychiatric field that INF-αoften causes a depressive state (1), a large-scale clinical study has indicated a risk of thyroid abnormalities from INF-α treatment (2). We encountered a patient who exhibited confusional delirium due to autoimmune thyroiditis after treatment with INF-α.

In this case the effect of INF-α itself on the central nervous system was not held responsible for the patient’s delirium because it did not appear during INF-α treatment but 10 days after treatment ceased and because thyroid hormone replacement therapy improved the patient’s symptoms. The thyroid side effect of INF-α treatment, which often requires long-term thyroid replacement therapy even after the cessation of treatment, is reported to appear frequently after 4 months of INF-α therapy because of an enhanced autoimmune mechanism (2).

Acquired hypothyroidism is reported to cause three types of psychiatric conditions that are historically categorized as myxedema madness (3): the hallucinatory-paranoid state, depression, and delirium (4). For this reason we considered the patient’s delirium to have been induced indirectly by autoimmunological processes in the thyroid gland due to repeated INF-α administration. Although immunosuppressants, including corticosteroids, cyclosporin A (5), and FK506 (6), are well known to cause delirium, the present case represents a possibility that INF-α, which is an immunoactivator, causes delirium by means of thyroid autoimmunological toxicity. Psychiatrists should consider the possible existence of a thyroid abnormality in patients with neuropsychiatric disorders induced by INF-α.