hypopituitarismPronunciation: /ˌhʌɪpəʊpɪˈtjuːɪt(ə)rɪz(ə)m/Definition of hypopituitarism noun [mass noun] Medicine diminished hormone secretion by the pituitary gland, causing dwarfism in children and premature ageing in adults. Derivatives hypopituitary adjective hypopituitarism in other Oxford dictionaries Definition of hypopituitarism in the US English dictionary