Most common hormone deficiency that occurs after intracranial irradiation is

The development of hypopituitarism occurs after 5-15 years and usually reflects hypothalamic damage rather than the absolute destruction of pituitary cells.Though the pattern of hormone loss is variable, Growth hormone deficiency is most common, followed by gonadotropins and ACTH deficiency.(Ref: Harrison's principals of Internal medicine, 16th Ed page 2078)