The following information may help to address your question:
Many things can cause syndrome of inappropriate antidiuretic hormone (SIADH), including brain injury, brain infection, brain abscesses, subarachnoid hemorrhage, encephalitis,
SIADH has also been reported in association with AIDS, temporal arteritis, polyarteritis nodosa, sarcoidosis, Rocky Mountain spotted fever, carcinoma of the cervix, olfactory neuroblastoma, and herpes zoster infection of the chest wall.
Often the underlying cause of the condition can not be determined. In these cases the condition is said to be idiopathic.