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The pituitary gland lies within a bony depression called the sella turcica within the sphenoid bone at the base of the brain. Its small size belies its many important functions. The pituitary gland has a central role in body growth, metabolism, and reproduction functions.
Normal Anatomy and Function
The pituitary gland consists of the anterior lobe (adenohypophysis), a pars intermedia, and the posterior lobe (neurohypophysis). The anterior lobe and pars intermedia are derived from Rathke's pouch. The anterior lobe is responsible for production and release of prolactin, adrenocorticotropin (ACTH), somatotropin (growth hormone), gonadotropin (GH), follicle stimulating hormone (FSH), luteinizing hormone (LH), and thyroid stimulating hormone (TSH). A portal-hypophysial system provides a direct vascular link between the median eminence of the hypothalamus and the anterior lobe. Releasing factors produced in the hypothalamus reach the pituitary through the portal system to direct release of the hormones.
The posterior lobe and infundibulum (pituitary stalk) are direct extensions of the hypothalamus. Vasopressin (ADH) and oxytocin are produced within neurons of the supraoptic and paraventricular nuclei and transported via axonal flow to the posterior lobe. The hormones are stored in secretory granules until their release into nearby capillaries.
The pituitary stalk transverses the suprasellar cisterns. The optic chiasm and nerves are located superior and anterior to the stalk and the supraclinoid carotid arteries are positioned laterally. The cavernous sinuses lie within the dura on either side of the pituitary gland and contain a meshwork of venous channels. The carotid arteries course through the middle of the sinuses. Cranial nerves 3 and 4 and the 1st (ophthalmic) and 2nd (maxillary) divisions of the 5th nerve are within the lateral walls of the cavernous sinuses. The 6th cranial nerve lies more medially within the sinus just below the cavernous carotid artery. The sphenoid sinus is immediately below the sella turcica.
The shape of the pituitary gland is variable but the gland height should not exceed 8 mm. The gland normally increases in size and upward convexity in adolescent females and during pregnancy.
The gland parenchyma is relatively homogeneous with standard spin-echo pulse sequences. However, very high resolution techniques will reveal a more heterogeneous texture. The anterior lobe is relatively isointense to gray matter on T1-weighted images except during the neonatal period when it is hyperintense. Endnote The normal posterior lobe is hyperintense due to T1 shortening by the phospholipid components of the neurosecretory vesicles Since the pituitary gland, stalk, and median eminence do not have a blood-brain barrier, they enhance routinely with gadolinium. The vascular plexus surrounding the stalk and the cavernous sinuses also enhance. The carotid arteries maintain a low-signal flow void.
Patients with diseases of the pituitary gland present with symptoms of endocrine dysfunction or with neurologic deficits from compression of adjacent structures. Mass lesions most often produce visual disturbances due to compression of the optic chiasm or optic nerves or a cranial neuropathy, but compression of the infundibulum can also be the cause of diabetes insipidus. The clinical presentation is helpful in differential diagnosis.
Abnormalities of Size and Position
Benign pituitary hyperplasia
As mentioned above, the pituitary gland normally enlarges around the time of birth, at puberty, and during pregnancy. Pathologic pituitary hyperplasia can be associated with a hyperfunctioning gland that mimics an adenoma. Symmetric enlargement and a homogeneous texture on plain and contrast scans help distinguish benign hyperplasia from a pituitary adenoma.
The empty sella
The empty sella results from herniation of arachnoid through an incompetent diaphragma sellae. Over time, CSF pulsations enlarge the sella and compress the gland against the floor of the sella. In most instances the empty sella is an incidental finding and asymptomatic. Occasionally, extreme compression of the gland may affect function, or traction upon the optic chiasm and nerves may cause visual symptoms.
During pregnancy the pituitary gland enlarges and becomes more susceptible to circulatory disturbances. Sheehan's syndrome or postpartum necrosis results from pituitary hemorrhage and infarction associated with complicated deliveries. Initial imaging studies will reveal a hemorrhagic gland in a normal sized sella. Over time, the gland involutes, resulting in a partially and completely empty sella.
The presence of high signal intensity adjacent to the median eminence with absence of the normal pituitary bright spot within the sella on T1-weighted images is evidence for the ectopic pituitary. Many cases are associated with perinatal asphyxia and disruption of the normal hypothalamic-pituitary axis. Traumatic transection of the stalk can also result in abnormal accumulation of posterior lobe hormones proximal to the disruption.