The first is an (almost) certain form where the etiology is known, for example if GHD is associated with cerebral or facial malformations, anatomic pituitary or hypothalamic abnormalities detected with magnetic resonance imaging (MRI), other pituitary hormone deficiencies, or established genetic causes of GHD, such as mutations of GH1, GHRHR or GHSR in isolated GHD (IGHD) and of multiple transcription factors in multiple pituitary hormone deficiency. Here, GHRHR is linked to hypopituitarism.