Hyperpituitarism refers to an endocrine disorder of the pituitary gland in which it secretes one or more of its hormones in excessive amount to the blood circulation. It is also a disorder of the pituitary gland which is characterized by excessive amount of all the hormones of the pituitary gland. In this article, There will be a discussion mainly of the causes of hyperpituitarism in addition to an overview of its function.
The pituitary gland is a small endocrine gland that is located at the base of the brain. It specializes in the secretion of several hormones to the blood circulation. The pituitary gland has two functionally distinct structures. These are called the anterior and the posterior pituitary glands. The pituitary gland is connected with the hypothalamus through a connection which is called the pituitary stalk.
The pituitary gland controls the function of many other endocrine glands in the body including the gonads in females and males in addition to controlling the adrenal and thyroid glands. Also the pituitary gland secretes a hormone which is important for the regulation of water in the body. This hormone is synthesized in the hypothalamus and is transported to the pituitary gland where it is stored until it is released based on a special stimulus. This stimulus is hyperosmolality of the blood or hypovolemia.
The anterior pituitary gland secretes hormones that all are regulated by a positive feedback from releasing hormones that are secreted by the hypothalamus. This is true except one hormone which is regulated by a negative feedback. This hormone is called prolactin and is secreted by special type of cells of the pituitary gland which are called lactotrophs. This hormone is under the negative feedback control from a hormone that is secreted by the hypothalamus and which is called dopamine.
All other hormones of the anterior pituitary gland are under positive feedback by specific hormones from the hypothalamus. All the hormones of the anterior pituitary gland have specific endocrine glands in the body which they control except one hormone which is called growth hormone. This hormone does not have a particular organ to act upon. Instead it acts in general on all body tissues to increase cellular growth and body maturation.
All hormones of the anterior pituitary gland have both negative as well as positive feedback control. An example is the hormone adrenocorticotropic hormone. This hormone is under positive feedback by the hormone corticotropin releasing hormone which is secreted by the hypothalamus. In addition it is under negative feedback from the hormone cortisol. High level of cortisol in the blood inhibits the secretion of adrenocorticotropic hormone by the pituitary gland. Growth hormone is both under positive as well as negative feedback from two hormones that are secreted by the hypothalamus.
Overactive pituitary gland usually secretes only one hormone in excess. The most common type of this disorder is hyperprolactinemia or increased secretion of the hormone prolactin into the blood circulation. This usually occurs due to an adenoma of the pituitary gland which oversecretes this hormone. This can lead to infertility in both men and women which are affected by this disorder.
Hyperprolactinemia can also occur in patients taking antagonists to the hormone dopamine such as the antipsychotic drug zyprexa. The inhibition of dopamine secretion by the effect of the drug lifts the negative feedback that is exerted on the secretion of prolactin, thus causing its excessive secretion.
The second most common type of hyperpituitarism occurs with growth hormone. This disorder can lead to accelerated growth in children leading to tall stature. Also this disorder can lead to diabetes mellitus or hyperglycemia. This is due to the stimulation of glycogenolysis by this hormone excess in the blood. In addition, adrenocorticotropic hormone oversecretion to the blood circulation can occur due to Addison’s disease in which the level of cortisol in the blood is diminished. This in turn lifts the negative feedback of this hormone on the pituitary gland, thus leading to excessive secretion of the adrenocorticotropic hormone.
Also in hypothyroidism or low level of thyroid hormone in the blood there is elevated level of the thyroid stimulating hormone observed due to the lack of the usual negative feedback that is exerted by the thyroid hormone on the pituitary gland secretion of the thyroid stimulating hormone.