The adenohypophysis, or anterior pituitary, is the forward lobe of the pituitary gland. As a component of the endocrine system, this small gland produces and releases multiple hormones that regulate a wide range of bodily functions, including growth, metabolism, and reproduction. Its activity influences several other endocrine glands, directing their functions and maintaining physiological balance.
Anatomy and Location of the Adenohypophysis
The adenohypophysis is part of the pituitary gland, a pea-sized structure at the base of the brain. It is housed within a bony depression in the sphenoid bone called the sella turcica. This location places it directly below the hypothalamus, to which it is physically and functionally connected. The adenohypophysis constitutes about 80% of the gland’s total weight and is anatomically distinct from the posterior lobe, or neurohypophysis.
The structure is composed of three parts: the pars distalis, pars tuberalis, and pars intermedia. The pars distalis is the largest portion and the primary site of hormone production. The pars tuberalis is a sheath that wraps around the connecting stalk, known as the infundibulum, while the pars intermedia forms a boundary between the anterior and posterior pituitary lobes.
Hormones of the Adenohypophysis
The adenohypophysis synthesizes and secretes six hormones that travel through the bloodstream to act on distant tissues and organs. Each hormone has a specific role in managing physiological processes, from metabolic functions to growth and reproductive cycles. The release of these hormones is a tightly regulated process.
Growth Hormone (GH), also known as somatotropin, primarily targets bones and muscles to promote growth and tissue maintenance. In children and adolescents, GH is a main driver of linear growth, while in adults, it helps maintain bone density, muscle mass, and influences fat distribution. It also has broader metabolic effects, such as stimulating protein synthesis and promoting the breakdown of fats for energy.
Thyroid-Stimulating Hormone (TSH) acts on the thyroid gland, a butterfly-shaped organ in the neck. TSH prompts the thyroid to produce and release its own hormones, thyroxine (T4) and triiodothyronine (T3). These thyroid hormones regulate the body’s overall metabolic rate, influencing everything from heart rate and body temperature to energy consumption at the cellular level.
Adrenocorticotropic Hormone (ACTH) targets the adrenal glands. It stimulates the outer part of the adrenal gland, the adrenal cortex, to produce cortisol. Cortisol is often called the “stress hormone” because its levels rise in response to stress, but it also plays a part in regulating blood pressure, blood sugar, and inflammation.
Two of the hormones, Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), are collectively known as gonadotropins because they act on the gonads (testes and ovaries). In females, FSH stimulates the development of ovarian follicles, which contain maturing eggs, and promotes estrogen production. LH triggers ovulation—the release of a mature egg—and stimulates the production of progesterone. In males, FSH is involved in sperm production, while LH prompts the testes to produce testosterone.
Prolactin (PRL) is primarily associated with lactation. During and after pregnancy, it stimulates the mammary glands to produce milk. Prolactin also has effects on reproductive function and can influence menstrual cycles in females and sexual function in both sexes. Outside of pregnancy, prolactin levels are low.
Regulation by the Hypothalamus
The secretory activity of the adenohypophysis is not autonomous; it is under the direct control of the hypothalamus. This relationship forms a neuroendocrine link known as the hypothalamic-pituitary axis, which ensures that hormone release is aligned with the body’s physiological needs.
This communication is facilitated by a specialized network of blood vessels called the hypophyseal portal system. This system of capillaries connects the hypothalamus directly to the anterior pituitary. Neurons in the hypothalamus release hormones into this portal system, which then transports them to the adenohypophysis to signal the pituitary cells.
The hypothalamus produces both releasing hormones and inhibiting hormones. For instance, Gonadotropin-releasing hormone (GnRH) stimulates the release of both LH and FSH, while Thyrotropin-releasing hormone (TRH) triggers the secretion of TSH. Conversely, inhibiting hormones put the brakes on pituitary secretion. Somatostatin, for example, inhibits the release of Growth Hormone, and dopamine acts to suppress the secretion of prolactin.
Disorders of the Adenohypophysis
Disruptions in the function of the adenohypophysis lead to medical conditions categorized by either the overproduction (hypersecretion) or underproduction (hyposecretion) of its hormones. These imbalances can have widespread effects on the body. Pituitary tumors, often benign growths known as adenomas, are a common cause of these disorders.
Hypersecretion occurs when the adenohypophysis produces an excess of one or more hormones. A common example involves Growth Hormone; its overproduction in adults leads to acromegaly, a condition characterized by the enlargement of bones in the hands, feet, and face. If excess GH occurs in childhood before the growth plates in bones have closed, it results in gigantism, leading to abnormal height. Another condition, Cushing’s disease, arises from the overproduction of ACTH, which in turn causes the adrenal glands to produce too much cortisol.
Hyposecretion involves a deficiency in hormone production, often caused by damage to the pituitary gland from tumors, injury, or blood loss. A deficiency in Growth Hormone during childhood can lead to pituitary dwarfism. In adults, a lack of hormones can cause symptoms like fatigue, low blood pressure, and reproductive issues. For example, a TSH deficiency can lead to an underactive thyroid, a condition known as central hypothyroidism.