rf has 8 distinct synonyms spread across 3 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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 substance produced by the hypothalamus that is capable of accelerating the secretion of a given hormone by the anterior pituitary gland
a radioactive transuranic element which has been synthesized
a complex neural network in the central core of the brainstem; monitors the state of the body and functions in such processes as arousal and sleep and attention and muscle tone
Synonyms, antonyms and the broader and narrower relations for rf come from Princeton University’s WordNet. Full sources and licences.