The posterior pituitary and diabetes insipidus.

SYNTHESIS Vasopressin is synthesised in hypothalamic neurones, predominantly concentrated in the supraoptic and paraventricular nuclei, which appear to be separate from those concerned in oxytocin synthesis. After synthesis, which may begin with the formation of a high molecular weight, biologically-inactive precursor molecule (Sachs and Takabatake, 1964), the vasopressin is packaged together with one or more neurophysins into neurosecretory granules (Sachs et al., 1969). These then pass down the axons of the neurons to reach the secretory terminal where vasopressin is released after appropriate stimulation. Secretory terminals are found not only in the posterior pituitary but also in the region of the median eminence. Because of the relatively wide anatomical extent of the vasopressin-producing system diabetes insipidus rarely results from disease of the pituitary itself. It is much more commonly seen arising either in the absence of any clinically or radiologically obvious cause or as a result of primary or secondary neoplasms (especially from breast) involving the hypothalamus. It may also be associated with granulomatous lesions of the hypothalamus-for example, histiocytosis X or sarcoidosis. Craniopharyngiomas or large suprasellar extensions of pituitary tumours may interfere with vasopressin release or synthesis, but more commonly diabetes insipidus results from the trauma associated with the treatment of such lesions.