Nonfunctioning pituitary tumors
1. Name the functioning pituitary tumors.
The normal pituitary gland secretes prolactin, growth hormone (GH), corticotropin (ACTH), thyrotropin (TSH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). The major functioning pituitary tumors are prolactin-secreting tumors, GH-secreting tumors, ACTH-secreting tumors, TSH-secreting tumors, and gonadotropin (FSH- and/or LH-)- secreting tumors. Some tumors secrete a mixture of hormones. These are all covered in other chapters.
2. What is a nonfunctioning pituitary tumor?
A nonfunctioning pituitary tumor arises from pituitary cells but does not secrete clinically detectable amounts of a pituitary hormone. These tumors are usually benign adenomas.
The alpha subunit is a component of three pituitary glycoprotein hormones: TSH, FSH, and LH. Each of these hormones consists of the common alpha subunit and a specific beta subunit (TSH beta, FSH beta, and LH beta). The alpha and beta subunits combine and become glycosylated before the intact hormone is secreted. Some nonfunctioning pituitary tumors secrete measurable amounts of the free alpha subunit, which may therefore serve as a tumor marker.
4. What other lesions can resemble nonfunctioning pituitary tumors?
Tumors that are not of pituitary origin may be found within the sella turcica; examples are metastatic carcinomas, craniopharyngiomas, meningiomas, and neural tumors. Nonneoplastic Rathke’s pouch cysts, arterial aneurysms, and infiltrative pituitary diseases, such as sarcoidosis, histiocytosis, tuberculosis, lymphocytic hypophysitis, and hemochromatosis, may also be seen.
5. Differentiate between a microadenoma and a macroadenoma.
A pituitary microadenoma is less than 10 mm in its largest dimension, whereas a macroadenoma is 10 mm or larger. A macroadenoma may be contained entirely within the sella turcica or may have extrasellar extension.
6. Which structures may be damaged by growth of a pituitary tumor outside the sella turcica?
Pituitary tumors that grow superiorly may compress the optic chiasm and pituitary stalk. Those that grow laterally can invade the cavernous sinuses and compress cranial nerves III, IV, and VI or the internal carotid artery. Inferior growth may erode into the sphenoid sinus. Anterior and posterior growth often erodes the bones of the tuberculum sellae and dorsum sellae, respectively (Fig. 19-1).
Figure 19-1. Pituitary fossa.
7. What are the clinical features of nonfunctioning pituitary tumors?
Nonfunctioning pituitary tumors are often asymptomatic and are discovered incidentally during cranial imaging procedures performed for other reasons. This is true of both microadenomas and macroadenomas. Tumors that cause symptoms are usually large, space-occupying macroadenomas, which compress nearby neurologic or vascular structures (see Fig. 19-1). Clinical features include headaches, visual field defects, visual loss, and extraocular nerve palsies. Pituitary insufficiency also may result from destruction of normal pituitary tissue.
8. What anatomic evaluation is necessary for a pituitary tumor?
Magnetic resonance imaging (MRI) or computed tomography (CT) of the pituitary gland and parasellar regions often allows a precise diagnosis and determines the presence and extent of extrasellar invasion. Visual field testing helps assess function of the optic chiasm and tracts. Angiography may be necessary in some cases to rule out an aneurysm.
9. What evaluation is necessary to determine that a pituitary tumor is nonfunctioning?
A thorough history and physical examination can detect symptoms and/or signs of pituitary hormone excess. Hormone testing should include measurement of serum prolactin, GH, insulin-like growth factor-1 (IGF-1), TSH, free thyroxine (free T4), LH, FSH, testosterone (men), estradiol (women), and 24-hour urinary cortisol excretion. Measurement of the serum alpha subunit is also helpful.
10. Does an elevated serum prolactin value indicate that a tumor is functioning?
No. Secretion of prolactin is negatively regulated by hypothalamic inhibitory factors, such as dopamine, which reach the anterior pituitary gland through the pituitary stalk. Stalk compression from a nonfunctioning tumor can impair dopamine delivery and thus increase the release of prolactin from the normal pituitary gland. The serum prolactin rarely exceeds 100 ng/mL in such cases, whereas it is usually much higher with prolactin-secreting tumors.
11. What is the natural history of nonfunctioning pituitary tumors?
