The pituitary gland is a small endocrine gland at the base of the brain that develops from two distinct embryological origins, which give it an anterior part, the adenohypophysis, and a posterior part, the neurohypophysis. The two parts are controlled in different ways, so each is described in turn, followed by how the size of the whole gland changes with age.
Adenohypophysis
The adenohypophysis lies anteriorly and makes its own hormones. Its output is under the control of releasing and inhibiting hormones that the hypothalamus releases into the hypothalamic-pituitary portal system, a small network of vessels that carries them down the pituitary stalk to the gland.
The hypothalamic-pituitary hormones are:
- CRH (corticotropin-releasing hormone) increases ACTH production.
- GHRH (growth hormone-releasing hormone) increases growth hormone production.
- GnRH (gonadotropin-releasing hormone) increases the production of FSH and LH.
- TRH (thyrotropin-releasing hormone) increases TSH and prolactin production.
- Somatostatin decreases growth hormone and TSH production.
- Dopamine decreases prolactin and TSH production.
Each anterior pituitary hormone answers to some of these signals, and several also answer to feedback or to a daily rhythm. The hormones produced by the adenohypophysis are:
- Growth hormone is released in response to GHRH; in healthy people it is also released in response to exercise, fasting, and stress. Its effects in the body are protein synthesis, lipolysis (breakdown of fat), and gluconeogenesis (the making of glucose).
- Prolactin release is inhibited by dopamine, and it has a trophic (growth-supporting) effect on the mammary glands. Because its dominant control is inhibitory, anything that interrupts the flow of dopamine down the stalk raises prolactin.
- Thyroid-stimulating hormone (TSH) release is under the control of feedback from thyroid hormone and also of TRH, dopamine, and somatostatin, and it follows a circadian clock, that is, a rhythm that repeats over the day.
- Adrenocorticotropic hormone (ACTH) production is under the control of CRH, a feedback mechanism, and also a circadian clock, which has its highest secretion during the morning. ACTH increases the production of adrenal cortex hormones (not those of the zona glomerulosa) and has a trophic effect on the adrenal gland.
- Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) act on the gonads: in men and women LH causes the formation of androgens and estrogens, while FSH is responsible for stimulating the process of gametogenesis, the production of egg and sperm cells.
Neurohypophysis
The neurohypophysis works differently from the anterior gland, because it does not make its own hormones. The hormones are produced by the hypothalamus and stored in the neuron endings inside the posterior hypophysis. The hormones released from the posterior pituitary gland are:
- Oxytocin is responsible for stimulating the smooth muscle cells of the uterus and mammary glands during delivery and breast-feeding, and during times when there is no breast-feeding it has a trophic effect on these organs.
- Antidiuretic hormone, or vasopressin, is released in response to an increase in plasma osmolality, and it also has a contracting effect on the arterioles, especially those in the kidney.
Gland height across life
The height of the pituitary gland is not fixed, and what counts as normal depends on the stage of life:
- During childhood it is about 6 mm.
- During adulthood it is about 8 mm, and the upper border of the gland is flat or slightly concave.
- During puberty and pregnancy it is about 10-12 mm, and the upper border of the gland is physiologically convex.
A gland this small, enclosed in the sella turcica and depending on signals from the hypothalamus above it, is affected both by processes that enlarge it and by processes that interrupt those signals.