Human Physiology - William F. Ganong 2002

Endocrine System, Metabolism, and Reproductive Function
Pituitary Gland
Hyperfunction of the Human Pituitary Gland

Acromegaly

Somatotropic Cell tumors of the anterior Pituitary gland secrete large amounts of Growth Hormone, leading to gigantism in children and acromegaly in adults. In 20–40% of patients with acromegaly, growth hormone hypersecretion is accompanied by prolactin hypersecretion. In addition, 37% of patients exhibit secretion of glycoprotein hormone α-subunits. Acromegaly can be caused by both extrapituitary and intrapituitary growth hormone-secreting tumors, as well as hypothalamic tumors that secrete growth hormone-releasing factor, although the latter are rare.

The MAIN CLINICAL MANIFESTATIONS of acromegaly (enlargement of the sella turcica, headaches, visual disturbances) are associated with the local mass effect of the tumor affecting growth hormone release. As noted above, there is an increase in the size of the hands and feet (acral parts, hence the term acromegaly) and forward protrusion of the Mandible (prognathism) (see Fig. 22-7). Excessive growth of the zygomatic, frontal, and facial bones combined with prognathism creates coarse facial features, referred to as the acromegalic facies.

In addition, body Hair increases. Skeletal changes predispose to The Development of osteoarthritis. Approximately 25% of patients have impaired glucose tolerance, and 4% present with galactorrhea.

Cushing's Syndrome

The Clinical presentation of Cushing's syndrome and its various causes are described in Chapter 20. Many patients with bilateral adrenal hyperplasia harbor small, occult ACTH-secreting pituitary tumors (microadenomas). In a significant percentage of patients, ACTH-secreting pituitary tumors grew rapidly following bilateral adrenalectomy (Nelson's syndrome). These tumors cause Skin hyperpigmentation and neurological signs resulting from pressure on structures within the sellar region. Some of these tumors are malignant. Blood ACTH concentrations are extremely high, and the MSH-like activity inherent in ACTH may account for the skin pigmentation. It remains unclear whether these patients had an undetected tumor initially or if neoplastic changes occurred in the pituitary when the negative feedback on ACTH secretion was removed.

Other Hormone-Secreting Tumors

In animals, TSH-secreting tumors occasionally develop following thyroidectomy, and gonadotropin-secreting tumors develop after gonadectomy. In humans, TSH-positive tumors are rare, whereas prolactin-positive tumors are quite common (see Chapter 23). In addition, many non-functioning pituitary tumors in women secrete gonadotrophins. Some women exhibit secretion of gonadotropin α- or β-subunits, or both simultaneously. The secretion of these compounds does not cause clinical disorders, but their elevated blood concentrations can be detected using appropriate Laboratory tests. Most, if not all, anterior pituitary tumors are monoclonal, originating from a single atypical cell.