The rule for definite articles and institutions is that if the speaker is at the institution as part of their normal life there's no article, but if they are going there for as special purpose then there is. So a student goes to school, but a parent goes to the school for a conference with the teacher. See also college, prison, church, university (in the UK). British English simply regards being a patient differently than American does. Here they still say they are going to the hospital to visit a patient, but that patient is in hospital.