The pituitary gland is a small structure located deep within the brain that serves as the central regulatory center of the endocrine system. This tiny organ governs bodily processes through the hormones it produces. Its influence extends to growth, metabolism, reproduction, and the function of other glands, including the thyroid and adrenals.
Anatomical Context and Location
The pituitary gland is situated at the base of the brain, positioned directly below the hypothalamus, which acts as its primary control center. The gland rests within the sella turcica, a protective bony cradle found in the sphenoid bone of the skull. It is organized into two primary regions: the anterior lobe (adenohypophysis) and the posterior lobe (neurohypophysis). The anterior lobe accounts for most of the gland’s mass and synthesizes and releases most pituitary hormones. The posterior lobe primarily stores and releases hormones produced by the hypothalamus, and this anatomical confinement means minor changes in size can significantly affect surrounding brain structures.
Standard Measurements and Variability
The normal adult pituitary gland is measured along three dimensions: length (anterior-posterior), width (transverse), and height (craniocaudal). An adult gland measures approximately 8 millimeters (mm) in length, 12 mm in width, and 6 to 9 mm in height. Height shows noticeable variation based on age and hormonal status; for instance, in men and postmenopausal women, height is usually 8 mm or less.
The gland’s dimensions change throughout life due to hormonal fluctuations. During adolescence, the gland temporarily increases in size, sometimes reaching 10 mm in height in females due to pubertal activity. The most pronounced growth occurs during pregnancy, where height can expand to 12 mm or more, primarily due to the multiplication of prolactin-producing cells. Following pregnancy and with increasing age, the gland gradually decreases in size.
The Role of Pituitary Size in Health
Monitoring pituitary size using magnetic resonance imaging (MRI) is a valuable clinical tool. Since the gland is encased within the sella turcica, expansion quickly leads to pressure on nearby structures. Its close proximity to the optic chiasm, where the optic nerves cross, means upward growth can compress these visual pathways. This compression may result in specific vision loss patterns, such as bitemporal hemianopsia, where peripheral vision is impaired on both sides.
A change in size marks potential dysfunction, prompting investigation into hormone levels and neurological symptoms. Clinicians use the gland’s morphology to differentiate between normal variations and pathological processes. Evaluating the superior surface—whether it is flat, concave, or convex—provides clues about functional status and potential lesions. A convex upper margin, for example, suggests an enlarged gland.
Conditions Affecting Pituitary Size
Pathological changes in pituitary size fall into two categories: enlargement or reduction. The most common cause of enlargement is a pituitary adenoma, an abnormal, non-cancerous growth of glandular cells. These tumors are classified by size: microadenomas measure less than 10 mm in diameter, and macroadenomas are 10 mm or larger. Microadenomas often cause symptoms through excessive hormone production.
Macroadenomas are more likely to cause symptoms through mass effect, compressing normal pituitary tissue and adjacent structures. Conversely, conditions causing size reduction often result in hypopituitarism, a loss of hormone production. Sheehan’s syndrome, a complication of severe postpartum hemorrhage, causes tissue death and atrophy, leading to a shrunken gland. Another cause is empty sella syndrome, where cerebrospinal fluid fills the sella turcica, causing the pituitary gland to flatten or appear compressed on imaging.