The hypophysis, more commonly known as the pituitary gland, is a small but powerful endocrine gland. Often referred to as the “master gland,” it holds a central position in the body’s intricate hormone system. This gland plays a coordinating role, directing other endocrine glands to release their hormones, thereby regulating growth, metabolism, blood pressure, and reproduction.
Anatomy and Location
The hypophysis is a pea-sized, oval-shaped structure, weighing between 0.5 and 1 gram. It is situated at the base of the brain, directly beneath the hypothalamus. This gland rests within a protective bony cavity known as the sella turcica, a depression found in the sphenoid bone.
The gland is composed of two primary parts: the anterior pituitary, also called the adenohypophysis, and the posterior pituitary, or neurohypophysis. The anterior pituitary is the larger section, making up about 80% of the gland’s total weight.
Hormonal Roles
The hypophysis produces and releases a wide array of hormones. The anterior pituitary synthesizes and secretes six major hormones. Growth Hormone (GH) promotes growth in children and helps maintain healthy muscles and bones in adults, also impacting fat distribution and metabolism. Thyroid-Stimulating Hormone (TSH) prompts the thyroid gland to produce thyroid hormones, which regulate metabolism, energy levels, and nervous system activity.
Adrenocorticotropic Hormone (ACTH) stimulates the adrenal glands to produce cortisol, often called the “stress hormone,” involved in metabolism, blood pressure, and inflammation regulation. Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), collectively known as gonadotropins, are important for reproductive function. FSH stimulates sperm production in males and egg development and estrogen production in females, while LH triggers testosterone production in males and ovulation and progesterone production in females. Prolactin (PRL) primarily stimulates breast milk production after childbirth and can influence fertility and sexual function.
The posterior pituitary does not produce hormones itself; instead, it stores and releases two hormones synthesized by the hypothalamus. Antidiuretic Hormone (ADH), also known as vasopressin, regulates water balance and sodium levels by controlling water reabsorption in the kidneys. Oxytocin plays a role in uterine contractions during childbirth and milk ejection during breastfeeding. It also influences social bonding.
Regulatory Mechanisms
The hypophysis is controlled by the hypothalamus, a region of the brain located directly above it. This connection integrates nervous system signals into hormonal responses. The hypothalamus communicates with the anterior pituitary through a specialized network of blood vessels called the hypophyseal portal system. Hypothalamic releasing and inhibiting hormones travel through this system to stimulate or suppress the release of anterior pituitary hormones.
For the posterior pituitary, the connection is neural, with nerve cells originating in the hypothalamus extending their axons directly into the posterior lobe. These nerve endings store and release ADH and oxytocin into the bloodstream. Negative feedback loops are important for this regulation. When hormones released by target glands reach sufficient levels, they signal back to the hypothalamus and hypophysis, inhibiting further hormone production and maintaining balance.
Disorders and Their Effects
Dysfunction of the hypophysis, whether producing too much (hypersecretion) or too little (hyposecretion) of its hormones, can lead to various health conditions. Imbalances in Growth Hormone (GH) can cause various effects. In children, excess GH before growth plates close leads to gigantism, characterized by unusually tall stature and enlarged features. Conversely, insufficient GH in childhood can result in dwarfism, leading to significantly shorter height. In adults, excess GH causes acromegaly, where bones become deformed and extremities enlarge, though height is not affected.
Disorders involving Adrenocorticotropic Hormone (ACTH) also have impacts. Overproduction of ACTH can lead to Cushing’s disease, a form of Cushing’s syndrome, characterized by symptoms like weight gain, high blood pressure, and changes in skin and mood, due to excessive cortisol. Conversely, a deficiency in ACTH can contribute to secondary adrenal insufficiency, where the adrenal glands do not produce enough cortisol, leading to fatigue, weakness, and low blood pressure.
Prolactinomas, which are noncancerous tumors on the hypophysis, cause excessive prolactin production. In females, this can result in irregular menstrual periods, infertility, and milky nipple discharge when not pregnant or breastfeeding. In males, it may lead to decreased libido, erectile dysfunction, and infertility.