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Follicle-stimulating hormone

Anterior Hormones Target Organs

Figure 13.13

Hormones released from the hypothalamus, the corresponding hormones released from the anterior lobe of the pituitary gland, and their target organs.

Thyroid-stimulating hormone, also called thyrotropin, is a glycoprotein. It controls secretion of certain hormones from the thyroid gland. TSH can also stimulate growth of the gland, and abnormally high TSH levels may lead to an enlarged thyroid gland, or goiter.

The hypothalamus partially regulates TSH secretion by producing thyrotropin-releasing hormone (TRH). Circulating thyroid hormones help regulate TSH secretion by inhibiting release of TRH and TSH; therefore, as the blood concentration of thyroid hormones increases, secretions of TRH and TSH decline (fig. 13.14).

External factors influence release of TRH and TSH. These include exposure to extreme cold, which is accompanied by increased hormonal secretion, and emotional stress, which sometimes increases hormonal secretion and other times decreases secretion.

U How does growth hormone affect the cellular metabolism of carbohydrates, fats, and proteins?

^9 What are the functions of prolactin in females? In males?

^9 How is TSH secretion regulated?

Adrenocorticotropic (ad-re"no-kor"te-ko-trop'ik) hormone is a peptide that controls the manufacture and secretion of certain hormones from the outer layer (cortex) of the adrenal gland. The secretion of ACTH is regulated in part by corticotropin-releasing hormone (CRH), which is released from the hypothalamus in response to decreased concentrations of adrenal cortical hormones. Stress can increase secretion of ACTH by stimulating release of CRH.

Growth Hormone Ups and Downs

Insufficient secretion of growth hormone during childhood produces hypopituitary dwarfism. Body proportions and mental development are normal, but because secretion of other anterior pituitary hormones is also below normal, additional hormone deficiency symptoms may appear. For example, a child with this condition often fails to develop adult sexual features unless he or she receives hormone therapy.

Human growth hormone, manufactured using recombinant DNA technology, is a valuable drug in treating hypopituitary dwarfism. Treatment must begin before the bones completely ossify. The hormone also has some controversial uses. Some people want to use it to increase height in children who are short, but not abnormally so. A few years ago, growth hormone was given experimentally to older individuals to see if it would slow aging-related changes. Although muscle tone improved and the participants reported feeling well, side effects arose, and the value of such treatment was not confirmed. In another application, bovine growth hormone is given to dairy cows to increase their milk production.

Oversecretion of growth hormone in childhood may result in gigantism, in which height may eventually exceed 8 feet. Gigantism is usually caused by a tumor of the pituitary gland, which secretes excess pituitary hormones and GH. As a result, a giant often suffers from other metabolic disturbances.

If growth hormone is over-secreted in an adult after the epiphy-ses of the long bones have ossified, the person does not grow taller. The soft tissues, however, continue to enlarge and the bones thicken, producing a large tongue, nose, hands and feet, and a protruding jaw. This condition, acromegaly, is also often associated with a pituitary tumor (fig. 13B). ■

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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