What Is the Pituitary Gland?
The pituitary gland, also referred to as the hypophysis, is a small, oval gland that is attached to the underside or base of the brain. It is an important link between the nervous system and endocrine system and releases many hormones that affect multiple body functions. Where Is the Pituitary Gland Located?
The pituitary gland is located on the underside of the brain within the skull. The pituitary is attached to the hypothalamus (an important part of the underside of the brain) via a stalk-like structure. The hypothalamus acts as the collecting center for information concerning the internal well being of the body. It uses much of this information to regulate the secretion of the hormones produced by the pituitary. What Is the General Structure of the Pituitary Gland?
The pituitary gland is no larger than the size of a pea, though its size varies among different dog breeds
and individuals. Physiologically, the pituitary gland is divided into two distinct lobes that arise from different embryological sources. The anterior (front) lobe is called the adenohypophysis, which is subdivided into three regions that produce a variety of hormones. It is controlled by substances called releasing hormones that are transported from the hypothalamus through tiny blood vessels.
The posterior (rear) lobe is called the neurohypophysis and is controlled by nerve fibers from the hypothalamus.
What Are the Functions of the Pituitary Gland?
Although very small, this organ plays a major regulatory role in the entire endocrine system. Each lobe of the pituitary gland produces certain hormones in response to the body's needs. They are then circulated in the blood to a variety of the body's tissues. The close structural positioning of the glandular and nervous segments of this gland is symbolic of its function in interrelating the nervous and endocrine systems.
The functions of the adenohypophysis are to make hormones that turn on other endocrine organs. Examples of these stimulating hormones are as follows:
Growth hormone stimulates growth of multiple cells and tissue types.
Prolactin stimulates milk production after giving birth.
Adrenocorticotropic hormone (ACTH) stimulates the adrenal glands.
Thyroid-stimulating hormone (TSH) stimulates the thyroid gland.
Follicle-stimulating hormone (FSH) stimulates the ovaries and testes.
Luteinizing hormone (LH) stimulates the ovaries and testes.
Melanocyte-stimulating hormone controls skin pigmentation or color.
The neurohypophysis has two major functions, depending upon the hormone produced:
Antidiuretic hormone (ADH) or vasopressin regulates water conservation by the kidney
Oxytocin is a potent stimulus of the muscles of the uterus and induces uterine contractions. It also encourages milk to be expressed from the alveoli into the mammary ducts during suckling.
What Are Common Diseases of the Pituitary Gland?
There are several disorders associated with the pituitary gland, most of which result from over production or underproduction of one of the pituitary hormones.
Hyperadrenocorticism. The most common disorder caused by overproduction of a pituitary hormone is hyperadrenocorticism or Cushing's disease. It develops from overproduction of ACTH, which stimulates the adrenal glands to make excessive amounts of cortisol hormones. Cushing's disease is usually seen in middle aged to older dogs. Excessive drinking, urinating, panting, hair loss and a pot-bellied appearance are common signs.
Diabetes insipidus. The most common disorder caused by underproduction of a pituitary hormone is diabetes insipidus or water diabetes. This is a disorder in which the kidneys are unable to retain water and is caused by the lack of the hormone ADH. Affected dogs cannot concentrate their urine and urinate excessively. They are also profoundly thirsty.
Rarer conditions that involve underproduction of pituitary hormones include failure to produce enough growth hormone. When growth hormone is deficient in young puppies, their growth is stunted and they are called pituitary dwarfs. When growth hormone deficiency occurs in the adult dog, the major clinical signs are abnormal hair growth.
Excessive production of growth hormone may occur in dogs given birth control-type medications on a chronic basis. This disease is called acromegaly and is very rare in the United States because these types of drugs are not commonly used in this country.
Pituitary tumors, both benign and malignant, may occur, but are not very common in dogs.
What Types of Diagnostic Tests Are Used to Evaluate the Pituitary Gland?
There are several tests that are particularly helpful in evaluating the pituitary gland.
Blood tests. Most of these tests measure the amount of cortisol in the blood stream and are timed tests that are taken over the course of a day. Examples of these include the ACTH stimulation test, the measurement of ACTH in the blood, and the low dose and high dose dexamethasone suppression tests (LDDST, HDDST).
A modified water deprivation test (withholding water until the urine becomes more concentrated) is sometimes helpful in diagnosing diabetes insipidus.
Serum (blood) growth hormone concentrations are useful in assessing disorders associated with the over or underproduction of growth hormone.
Computed tomography (CT scan) and/or magnetic resonance imaging (MRI) are also valuable tools in assessing the shape, size and internal structure of the pituitary gland.