In progressive hypopituitarism, for example due to an anterior pituitary tumour, there is a characteristic order in the development of hormone deficiency (Fig. 2.9). Usually, secretion of GH and LH fails first; this is followed later by failure of FSH and TSH secretion, and finally by that of ACTH secretion. Prolactin deficiency is rare except in Sheehan's syndrome, where it is associated with a failure of lactation. Antidiuretic hormone (ADH or vasopressin) deficiency is virtually unknown in patients with anterior pituitary tumours, although it may occasionally be seen in patients following head injury. The result is diabetes insipidus.
