The pituitary gland is a pea-sized endocrine gland that sits at the base of the brain. Often referred to as the “master gland”, the pituitary gland synthesizes and releases various hormones that affect several organs throughout the body (see the images below).
Hormones secreted by adenohypophysis (anterior pituitary).
Hormones secreted by neurohypophysis (posterior pituitary).The fully developed pituitary gland (see the image below) is pea-sized and weighs approximately 0.5 g. The adenohypophysis constitutes roughly 80% of the pituitary and manufactures an array of peptide hormones. The release of these pituitary hormones is mediated by hypothalamic neurohormones that are secreted from the median eminence (a site where axon terminals emanate from the hypothalamus) and that reach the adenohypophysis via a portal venous system.
Pituitary gland, sagittal section.The pars distalis forms the majority of the adenohypophysis and resembles a typical endocrine gland. Cords and clusters of cuboidal secretory cells within the pars distalis contain hormones stored in cytoplasmic granules that are released via exocytosis and taken up by nearby sinusoidal capillaries. Histochemical staining of these granules with pH-dependent dyes allows categorization of the cells into acidophils, basophils, or chromophobes.
In general, acidophilic cells contain polypeptide hormones, basophilic cells contain glycoprotein hormones, and chromophobes have minimal to no hormone content. The most common cell type is the acidophilic somatotrope, which is concentrated in the lateral regions of the adenohypophysis and secretes growth hormone (GH). Lactotropes are also acidophilic but are more scattered throughout the adenohypophysis and secrete prolactin (PRL).
The basophilic cells include corticotropes, thyrotropes, and gonadotropes. Although corticotropes secrete nonglycosylated polypeptides such as adrenocorticotropic hormone (ACTH), these cells are basophilic as a result of the glycoprotein composition of the precursor hormone pro-opiomelanocortin (POMC). Thyrotropes are among the least prevalent secretory cells of the pars distalis; they release thyroid-stimulating hormone (TSH), whereas gonadotropes secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH).
Pituitary tumors are relatively common, accounting for about 15% of all primary brain tumors. [21] The vast majority originate in the adenohypophysis and are typically nonsecretory benign adenomas. These adenomas frequently go undiagnosed, and meta-analyses of postmortem studies have demonstrated an 11-14% overall prevalence of silent pituitary adenomas in the general population. [22, 23] Tumors of the neurohypophysis are rare and include metastasis, granular cell tumors, and potentially any primary tumor of the neuraxis.
Pituitary adenomas are arbitrarily classified as microadenomas (< 1 cm) or macroadenomas (> 1 cm). Macroadenomas, when large, have a mass effect on adjacent structures, with clinical consequences. Compression of the pituitary gland itself may cause hypopituitarism, and compression of the optic chiasm results in bitemporal hemianopsia. Headache is also a common symptom of pituitary tumors.
Secretory adenomas are typically monoclonal–that is, they secrete a single hormone. Approximately 1 to 2% of adenomas secrete 2 or more hormones, with growth hormone (GH) and prolactin (PRL) being the hormones most commonly elevated concomitantly.
Diagnostic imaging
Magnetic resonance imaging (MRI) is the study of choice for evaluating the pituitary gland. [33, 34] This multiplanar imaging modality has the advantages of providing superior contrast differentiation of soft tissues and not exposing the subject to potentially harmful ionizing radiation. Coronal and sagittal T1-weighted sequences with 3 mm thick sections are typically recommended for detecting pituitary lesions.[35, 36] As a supplement, T2-weighted images are often useful.
Hyperintense signals on T1-weighted images can be due to numerous disease processes, such as hemorrhage, Rathke’s cleft cyst, and craniopharyngioma. However, several normal conditions, such as vasopressin storage in the posterior lobe, can also present as hyperintensities. [37] Microadenomas commonly appear hypointense on noncontrast T1-weighted images but can occasionally appear isointense.
The sensitivity of lesion detection with MRI can be improved by repeating T1-weighted sequences after administration of a gadolinium-containing contrast agent.[38] Although a single dose (0.1 mmol/kg) of gadolinium (Magnevist, Berlex, Wayne, NJ) is effective, a half dose and a double dose have also been demonstrated to be advantageous. [39, 40, 41]
In the setting of an intact blood-brain barrier, the normal pituitary gland and infundibulum present with homogeneous contrast enhancement, whereas the hypothalamus and optic chiasm remain unaffected. Because of temporal variations in the enhancement patterns of lesions, dynamic MRI after intravenous (IV) gadolinium bolus injection can potentially provide additional valuable information.