Hypothalamic and Pituitary Disorders
Anatomy and hormone loss
The pituitary sits in the sella, connected to the hypothalamus by the stalk. Anterior hormones are GH, LH/FSH, TSH, ACTH, and prolactin; posterior hormones are ADH and oxytocin. Causes of hypopituitarism include pituitary stalk interruption syndrome, with a small anterior pituitary, thin or absent stalk, and ectopic posterior pituitary; septo-optic dysplasia, with optic nerve hypoplasia and midline defects; holoprosencephaly; craniopharyngioma; radiotherapy; hypophysitis; trauma; and infiltrative disease. In acquired disease, hormone loss usually follows GH, then gonadotropins, then TSH, then ACTH; prolactin may rise with stalk compression. Neonatal clues are hypoglycemia, prolonged jaundice, micropenis, and undescended testes. Central adrenal insufficiency spares aldosterone, so hyperkalemia is not expected. Central hypothyroidism shows low free T4 with low or normal TSH.
Traps and workup
Do not diagnose GH deficiency from a single random GH. Assess baseline free T4/TSH, IGF-1, prolactin, and morning cortisol, and perform dynamic testing when safe. In suspected central diabetes insipidus, water deprivation shows inability to concentrate urine with response to desmopressin; untreated ACTH deficiency can mask diabetes insipidus by impairing free-water clearance. MRI is essential. After craniopharyngioma surgery, watch for the triphasic response: transient diabetes insipidus, SIADH, then permanent diabetes insipidus.
Rule of thumb
If a child has hypoglycemia plus micropenis or midline facial defects, screen for hypopituitarism before assuming isolated GH deficiency. Treat cortisol before thyroid hormone when both are deficient, because starting levothyroxine can precipitate adrenal crisis.