WordNet records no synonym and no antonym for referral. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a person whose case has been referred to a specialist or professional group
“the patient is a referral from Dr. Bones”
a recommendation to consult the (professional) person or group to whom one has been referred
“the insurance company says that you need a written referral from your physician before seeing a specialist”
the act of referring (as forwarding an applicant for employment or referring a matter to an appropriate agency)
Synonyms, antonyms and the broader and narrower relations for referral come from Princeton University’s WordNet. Full sources and licences.