The Anatomy of the Pituitary Gland

Known as the master gland, the pituitary gland is behind the bridge of the nose.

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About the size of a small pea, the pituitary gland, also known as “the master gland,” plays a crucial role in regulating hormone production from most of the other glands in the body. This being the case, it’s essential to numerous functions as well as overall health. Sitting in the brain between its hypothalamus region and the pineal gland within the sphenoid bone (located towards the front of the skull), this gland has two lobes: an anterior and a posterior lobe.    

Given its critical role, diseases or malformations of the pituitary gland can have serious implications. These include often asymptomatic pituitary tumors (adenomas), Cushing’s disease (caused by excess steroid use), and hypopituitarism, characterized by underactivity of the gland.

Anatomy

Structure

The pea-sized pituitary gland is composed of both anterior and posterior lobes; in adults, the vertical diameter is approximately 8mm, with the horizontal circumference found to be 12 millimeters (mm). These are encased in a tough membrane (dura), and just beneath another such membrane, the sellar diaphragm, which has an opening to allow a structure called the infundibular stalk to exit the gland.    

Each of these lobes has sub-parts and structures. Here’s a quick breakdown of these:

  • Anterior pituitary lobe: This front-facing portion is the largest of the pituitary gland. The anterior pituitary lobe is responsible for the synthesis of most pituitary hormones. It consists of the pars distalis, a structure composed of strings of specialized cells that secrete hormones associated with growth and development (trophic hormones). The pars tuberalis is a part that surrounds the infundibular stalk, and the pars intermedia is a thin band of cells that separates the pars distalis from the posterior pituitary lobe.
  • Posterior pituitary lobe: The rear-facing lobe of the gland is an extension of the hypothalamus brain region that is connected to the main body via the infundibular stalk, which is itself considered a part of the posterior pituitary lobe. This stalk runs from the tuber cinereum, a hollowed-out eminence of the hypothalamus, to pierce the sellar diaphragm. 

Location

The pituitary gland rests in a saddle-shaped depression in the middle of the sphenoid bone called the sella turcica. This butterfly-shaped, unpaired bone is located towards the front of the skull at about eye level. This places it just beneath the optic chiasm (where the optic nerves cross), the hypothalamus, as well as the front portion of a ring of arteries called the circle of Willis. It’s to the side of the cavernous sinus, a space that collects blood from central brain regions on its way back to the heart. To the front of the pituitary gland, you find a couple of other blood-collecting spaces—the anterior clinoid and anterior intercavernous sinuses.         

Anatomical Variations

Several congenital variations occur with the pituitary gland. Among the most notable of these is that there is variation in size between men and women, with these being somewhat larger in the latter. Pregnancy also causes this gland to grow significantly in size. Similarly, the pituitary gland is larger during puberty and young adulthood, and it’s known to shrink after age 50.

In addition, a number of other anatomical differences have been observed by doctors. These include:

  • Hypoplasia: This is an under-development of the anterior lobe of the pituitary gland, which can severely affect its function.
  • Hyperplasia: Excessive enlargement of the pituitary gland sometimes occurs during pregnancy or in young, menstruating women.
  • Partially empty sella turcica: A variant of empty sella, this is a relatively common condition, in which the sella turcica portion of the pituitary gland is empty and flattened.
  • Duplication: In extremely rare cases—and usually alongside other congenital issues—the pituitary gland may be duplicated. Most reported cases occurred in women or girls and are associated with facial or cranial birth defects. 

Function

Given its instrumental role in the body, the pituitary gland is highly influential on human development and functioning. Primarily, this is done through the synthesis of hormones. As mentioned above, the anterior lobe is the site of the majority of such activity and produces the following:

  • Adrenocorticotropic hormone (ACTH): When corticotropin-releasing hormone (CRH) is released from the hypothalamus and reaches a specific area, where it divides into several hormones, including ACTH. These travel to the adrenal cortex (on top of the two adrenal glands, located on top of the kidneys), and then travel in the bloodstream to release cortisol. In turn, cortisol regulates the secretion of glucocorticoids in periods of stress
  • Prolactin (PRL): Regulated directly by the hypothalamus, PRL is directly associated with the growth of mammary glands to start producing milk in women. Its activity is inhibited by the brain chemical, dopamine, and in post-partum mothers, this chemical is inhibited when babies nurse. This, in turn, stimulates prolactin activity, and therefore lactation.
  • Luteinizing hormone (LH) and follicle-stimulating hormone (FSH): Gonadotropin-releasing hormone (GnRH) is released from the hypothalamus to stimulate the development of LH and FSH. In men, LH acts on specific cells in the testes (Leydig cells) to produce testosterone, and FSH acts on other cells (Sertoli cells) to take part in the development of sperm. In women, LH causes ovaries to produce steroid hormones, which in turn is involved in ovulation. FSH works on the cells associated with developing female gametes (called granulosa cells), which are cells that can be fertilized to become zygotes.     
  • Growth hormone or somatotropin (GH): This stimulates cell growth throughout the body and is regulated by a feedback loop based on levels of this hormone in the blood.
  • Thyroid-stimulating hormone (TSH): This hormone stimulates the thyroid gland to release T3 and T4—hormones that regulate metabolism in every cell in the body.

In addition, the posterior pituitary lobe synthesizes a couple of other hormones, which are:

  • Oxytocin: This hormone is most commonly associated with social and sexual bonding, which is why it’s sometimes referred to as “the cuddle hormone.” In pregnant women, secretion of this substance causes contractions leading to labor, and, in the post-partum period, it causes the milk let-down reflex, which is the release of breastmilk when the baby latches on to feed.   
  • Arginine vasopressin (AVP) or antidiuretic hormone (ADH): This hormone serves several important functions, including water regulation and water depletion in the body, as well as regulation of blood pressure in cases of blood loss. AVP causes arteries to contract via special receptors throughout the body, and, by acting on the kidneys and interacting with a protein called aquaporin 2, it creates channels to help water reabsorb into the bloodstream.  

Associated Conditions

A number of conditions and diseases can affect the pituitary gland: everything from infection or inflammation to the presence of tumors. A majority of the problems here are related to the latter case, and these are typically treated using either gamma knife radiosurgery, which employs directed radiation to perform surgery, another type of radiotherapy called intensity-modulated radiation therapy (IMRT), or, in some cases, traditional surgery. Here's a quick breakdown:

  • Pituitary adenoma: Adenomas are tumors that grow on the pituitary gland. Almost always benign (non-cancerous), these occur in about 20% of people and in many cases are asymptomatic. Their presence may be associated with other health conditions, such as high blood calcium level. These adenomas—due to their size—lead to under-activity of the gland or over-production of hormone (also known as hypopituitarism). Occasionally, these adenomas lead to headaches or vision problems.
  • Hyperprolactinemia: This type of tumor causes the pituitary gland to produce the hormone, prolactin. Varying in size, with smaller ones called “microprolactinomas” and larger growths called “macroprolactinomas,” these can lead to discharges from the breasts in women, irregular menstruation, or even loss of menstrual function in women. In men, this condition can lead to impotence. Occasionally, these grow large enough to provoke symptoms. 
  • Pituitary apoplexy: This is a rare condition, in which a pituitary adenoma enlarges in size and starts to take on arterial blood, leading to obstruction of blood flow. In turn, this leads to sudden headache, visual disturbances, reduced hormone production, and, in some cases, vomiting.
  • Cushing’s syndrome: Often the result of overexposure to steroids—though also occurring in cases where adenomas are causing hyperactivity of hormone production—Cushing’s syndrome leads to overactivity of adrenal glands, leading to overproduction of cortisol. More common in women, this condition leads to progressive weight gain, depression, muscle weakness, and easy bruising of the skin. In men, it can lead to impotence, and in women, it can cause irregular periods.
  • Hypopituitarism and panhypopituitarism: Hypopituitarism is the situation in which the pituitary gland is not producing certain hormones, which can lead to panhypopituitarism, or an underproduction of hormones from other glands. As with other conditions, this is the result of benign tumors impacting the anterior or peripheral lobes, or it can arise as an unintended side-effect of surgery. Occasionally, these arise due to infection or certain head injuries. Symptoms include tiredness, irregular periods or even complete loss of menstrual function in women, impotence (in men), infertility, susceptibility to cold temperatures, constipation, dry skin, and low blood pressure. 

Tests

If you complain of symptoms associated with pituitary dysfunction, your doctor will first need to take a look at your medical history. This means that you’ll need to have any imaging or test results on hand for the consultation. If the situation calls for it, the pituitary gland can be assessed using a number of specialized approaches, including:

  • Insulin tolerance test: Used to test adrenal and pituitary gland function—and a common test for diabetes—this procedure involves administering insulin to induce hypoglycemia, or lower blood sugar. This allows the doctor to assess how well this gland is able to produce necessary hormones. 
  • Dexamethasone suppression test: This assesses the response of adrenal glands to ACTH by measuring cortisol levels in urine. Basically, it aims to assess whether the pituitary gland is ensuring the right amount of cortisol is being produced. In particular, high dose versions of this test ascertain the presence of Cushing’s syndrome. 
  • Growth hormone stimulation test (GHRH): Also known as the arginine test, GHRH assesses the level of growth hormone (GH) production. This involves drawing blood and applying medicine to stimulate pituitary function and measuring these levels.   
  • Growth hormone suppression test: This tests for conditions involving overactive pituitary function, such as Cushing’s syndrome. By suppressing growth hormone production using specific drugs, doctors can assess for GH deficiency as well as hypopituitarism. 
  • Magnetic resonance imaging (MRI): After initial tests, doctors may require imaging—often MRIs—to get a fuller sense of pituitary health and assess for the presence of any tumors.   
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