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Chapter 11: The hypothalamus and pituitary 421

Thyroid Stimulating Thyroid Hormone
Anterior Pituitary Gland Thyroid Gland Target Organs
Hormone +
−

Figure 11.1 An example of negative feedback control.

corticotrophin-releasing hormone and thyrotrophin–
The hypothalamus and pituitary releasing hormone have an important circadian (circa-
about, dia -day)rhythm.
The hypothalamus and pituitary form the basis of the
Introduction to the hypothalamus central control of various endocrine axes, which are vital
and pituitary to everyday function (see Fig. 11.2). Disorders of the
hypothalamus itself are very rare; however, disorders of
The pituitary gland lies in the sella turcica, which is a
tightly enclosed bony space at the base of the cranium, the pituitary are common.
roofed by a reflection of the dura. The optic chiasm lies
just above the pituitary fossa and the cavernous sinuses Pituitary adenomas
run lateral to it. These structures may be affected by
expansion of the pituitary gland. It consists of two lobes: Definition
The posterior lobe is a physical and functional exten-
Pituitary adenomas are benign slow growing tumours
sion of the ventral hypothalamus. The nerve endings arising from the anterior pituitary.
within the posterior pituitary contain and secrete oxy-
tocin and vasopression (antidiuretic hormone). Aetiology
The anterior lobe originates from Rathke’s pouch. Al-
The cause of most pituitary adenomas is unknown. Gene
though the anterior lobe is of separate origin to the hy- mutationshavebeencharacterisedinsomepituitaryade-
pothalamus,itisunderitsclosecontrol.Hypothalamic nomas, for example in the condition multiple endocrine
hormones reach the anterior pituitary in high concen- neoplasia (MEN) type 1 tumours including pituitary
trations via hypophyseal–pituitary portal veins. adenomas occur due to the loss of tumour suppressor
The hypothalamus lies just above the pituitary, and has genes.
centres for appetite (the satiety centre), thirst, tem-
perature control and the sleep–wake cycle. The hy- Pathophysiology
pothalamussecretespolypeptidehormonesthatregulate Seventy per cent of pituitary adenomas are functioning,
anterior pituitary hormone secretion, mostly by stim- i.e.hormonesecreting.Thesetendtopresentearlierthan
ulation. They are secreted episodically and some, e.g. the other 30% non-functioning tumours.

Hypothalamus CRH GHRH Somatostatin TRH Dopamine GnRH
Corticotrophin GH Releasing Thyrotrophin Gonadotrophin
Releasing Hormone Hormone Releasing Hormone Releasing Hormone
+ + − + − +
Anterior
ACTH Growth Hormone TSH PRL LH / FSH
Pituitary
Adrenal Axis Growth Axis Thyroid Axis Dopamine and Gonadal Axis
Prolactin Axis
Figure 11.2 Hypothalamic and pituitary secretion.