Dr. Saima Amer, Endocrinologist
Department of Diabetes & Endocrinology, Gold Coast Hospital
- Feed Back Loops
- Pituitary Hormones
- Anterior Pituitary Hormones
- Posterior Pituitary Hormones
- Are there other Endocrine glands in the body?
- How would I know if my pituitary gland was not functioning correctly?
The pituitary gland is the “master gland” of the body. It controls the functioning of most of the other endocrine organs of the body called “glands”. Glands are organs that produce hormones.
Hormones are chemical messengers that travel in the blood stream to reach their specific target organs, which are affected by the action of the hormone. This sounds funny. How is it that the hormones only act on certain organs/tissues when in fact, through the blood stream, they can reach every tissue of the body? To understand this, consider the analogy of a lock and key. The key would open only the lock it has been designed for. Hormones work on their target tissues or cells in the same way. They attach to receptors on the surface of the cells, and bring about a series of changes in the target cell.
The pituitary gland sits very securely in a bony pocket of the skull called the “sella turcica”. The gland is about the size of a pea. It is actually comprised of two parts. The anterior pituitary is the front part, and the posterior pituitary is the hind part of the pituitary gland. The posterior pituitary is an actual extension of neurons from the under-surface of the brain called the hypothalamus.
The function of the pituitary gland is controlled by two types of influences; mainly by the lower part of the brain called the “hypothalamus”, and via a “feed-back loop” by the hormones made by other glands of the body.
A feedback loop is also an interesting and important concept to understand in order to facilitate the understanding of endocrine systems. This can be best demonstrated by the example of thyroid hormones (T3 and T4) and their relationship with the pituitary hormone (TSH) which stimulates the production of thyroid hormones.
The hypothalamus, which is the lower part of the brain, produces a hormone called TRH (Thyrotrophin Releasing Hormone). TRH stimulates the pituitary gland to make TSH (Thyroid Stimulating Hormone) which, as its name suggests, stimulates the thyroid gland to make Thyroxine (T4). When the levels of T4 and also T3 (an active form of T4) exceed a certain limit, T4 and T3 have an inhibitory (negative) effect on both the hypothalamus as well as the pituitary gland, so that they reduce the production of TRH and TSH. This is in turn results in reduced production of T4 and T3 by the thyroid gland. When the levels of T4 and T3 drop below a certain limit, the negative effect on the hypothalamus and the pituitary goes away, resulting in increased production of both of these hormones, which then increase T4 and T3 production. This loop is orchestrated to maintain normal levels of thyroid hormones in normal individuals. Any increases or decreases are designed to meet the demands of the body at certain times. Most of us would be glad to know we don’t have to consciously think about it on a day-to-day, and hour-to-hour basis.
The hormones secreted by the pituitary are generally studied best by dividing them up between those secreted by the anterior pituitary and the posterior pituitary.
Also, to understand these hormones better, we need to understand what the target glands do in response to pituitary hormones. Generally there is a cascade of hormones that is secreted. Hormones secreted by the hypothalamus control the pituitary hormones, and the pituitary hormones in turn control hormones secreted by other glands of the body. Those hormones in turn go and act on different cells and tissues of the body and cause a stream of reactions or actions that lead to some changes in the body function, be it growth, heart rate, glucose levels in blood, secretion of milk for the baby, or contraction of the uterus during labour.
The anterior pituitary makes the following hormones:
- GH (Growth Hormone)
- TSH (Thyroid Stimulating Hormone)
- ACTH (Adrenocorticotrophic Hormone)
- FSH (Follicle Stimulating Hormone)
- LH (Luteinizing Hormone)
The posterior pituitary makes the following hormones:
- Anti-Diuretic hormone (ADH, also called Vasopressin)
Growth hormone is secreted by pituitary cells called Somatotrophs. It is a peptide hormone. The secretion of growth hormone, like a few others is pulsatile and follows a circadian or day-night rhythm. GH levels are highest at night-time when we are asleep, and relatively lower during the day. GH secretion varies at different stages of life. Adolescents have the highest levels, followed by children, and then adults. As we age the GH levels fall off and there has been much interest in GH as an anti-aging hormone, but so far there has been no proof that it can reverse or stop the clock. GH is ketogenic, i.e. it stimulates the formation of free fatty acids, which can be used by the body as a fuel when it is deprived of glucose as a source of energy.
As the name indicates, the main function of growth hormone in children and adolescents is regulation of growth. GH does this by recruiting other hormones; mainly IGF-1 (Insulin-like Growth Factor-1) which is secreted by the liver in response to GH. There are many others in the family, but IGF-1 is by far the most important.
Thyroid Stimulating Hormone
Human TSH is a glycoprotein. It stimulates the thyroid gland to produce thyroxine, which is a hormone that has wide spread implications for metabolism in the body. Absence of thyroxine is incompatible with life. Patients with pituitary problems, who lose the function of TSH, have to have thyroxine replacement life-long. Thyroid hormone or thyroxine increases mental alertness, heart rate, motion of the gastrointestinal tract, breathing drive, as well as brain development and skeletal maturation in the foetus.
ACTH (Adreno Corticotrophic Hormone)
ACTH is secreted by pituitary cells called corticotrophs. The hormone stimulates the adrenal glands to make cortisol, which is the natural steroid our bodies make. The adrenal glands are two small glands, one on top of each of our kidneys. They make other hormones in addition to cortisol, but for the purposes of this article we will not discuss those hormones as they are not under the control of the pituitary gland. Cortisol is essential for survival; absence of cortisol is incompatible with life.
One of the interesting, as well as very important aspects of steroid secretion is that steroids are secreted by the adrenal glands in the form of pulses, in a circadian rhythm. The maximum secretion of steroids happens around the early hours of the morning and reaches a peak generally when we wake up. Afterwards the levels of steroids slowly come down during the day as the sun goes down. Changes in ACTH secretion through the day govern this day-night cycle of steroid secretion.
Note that patients who take steroids due to pituitary or other diseases have their maximum dose first thing in the morning, and progressively lesser doses during the day. This is because we are trying to mimic the natural cycle.
Steroids maintain normal blood pressure, increase protein and fat metabolism, and increase blood glucose levels. Externally administered steroids can make diabetes worse. They generally suppress the immune system. They can stop the bone-building cells, resulting in osteoporosis, a condition resulting in weak bones and susceptibility to fractures due to a low level of trauma. Steroids have effects on mood, and play important roles in growth and development. Different factors can have an impact on the amount of steroids the body needs or secretes. Stress, both physical and emotional, results in increased steroid levels. Infection, inflammation and low blood glucose levels can all increase the steroid production by the body, provided all the feedback circuits are working normally. In patients with a deficiency of ACTH, the ability to increase steroid production is absent, therefore patients are asked to increase the dose of their steroids when they are ill or have elective surgery etc.
FSH (Follicle Stimulating Hormone) and LH (Luteinizing Hormone)
These two hormones are best described together as they have actions very closely related. These are stimulated by a hypothalamic hormone called GnRH (Gonadotrophin Releasing Hormone). These control the ovarian function, formation of follicles and hence eggs (ova), and maintenance of the female reproductive cycle in females. They stimulate formation of testosterone and sperm production in males. In females FSH stimulates the development of the follicles in a cyclical manner. The follicle then produces oestrogen. In men FSH stimulates the production of sperm. LH is responsible for causing ovulation in females, and in men production of testosterone.
Vasopressin (Anti-Diuretic Hormone, ADH)
It is secreted by the pituitary cells called lactotrophs, and causes milk secretion from the breast after oestrogen and progesterone priming during pregnancy. Prolactin also inhibits the secretion of gonadotrophins (the hormone responsible for pituitary secretion of FSH and LH). Therefore some patients with prolactin-secreting tumours can present with irregular cycles, or infertility, or low testosterone levels. Its secretion is increased by exercise, surgical and psychological stresses, and stimulation of the nipple. Secretion is increased during pregnancy, reaching a peak at the time of child birth. Interestingly prolactin is under an inhibitory control, i.e. dopamine. This factor secreted by the hypothalamus inhibits the secretion of prolactin. Hence, if due to a tumour the pituitary stalk is compressed, prolactin levels rise as the inhibitory signal is removed.
ADH is secreted in response to high serum osmolality (Na) or low blood pressure. It works on the kidneys to conserve water, hence restoring osmolality. It also works on the blood vessels to increase blood pressure.
It is released during labour and increases uterine contractions at the time of delivery. It also causes the ejection of milk from the breast by causing contraction of the myo-epithelial cells. It is the hormone that is used in the form of a drip to induce labour in women who do not enter into labour naturally, and is commonly known as a “Syntocinon” drip.
Yes. Other glands in the body include the four parathyroid glands for calcium production, the pancreas gland for insulin production, and the central parts of the adrenal glands which produce adrenalin and nor adrenaline. These glands do not need the pituitary gland to give them messages to make their hormones, they are dependant on other chemical or neurological control, but they are part of the endocrine system.
Pituitary gland hormones, through their target glands and hormones secreted from the target glands, control a wide range of body functions. Most of these are essential for normal growth, metabolism and reproduction. Deficiency or excess of these hormones can therefore lead to a wide range of problems.
There are a variety of things which may happen. Usually more than one physical sign or sympton (feeling) may be found.
In children and adolescents, linear growth and puberty are dependant on hormal signals from the hypothalamus and pituitary gland, and therefore problems in this hormal axis may become manifest as problems in linear growth and acquisition of puberty.
In women of reproductive age, problems with monthly cycles, and fertility can be the first sign of pituitary problems. A wide variety of symptoms including but not limited to weight and appetite changes, and changes in skin, vision, mood, general well being can be due to pituitary problems. These signs and symptoms may occur because of the target organs not working effectively and not be the fault of the pituitary gland itself.
The expression “we are slaves to our hormones” is one that people with an imbalance of the endocrine system can relate to very well. Your GP would generally look at more common problems first before considering any pituitary problem as the underlying reason. Your endocrinologist (hormone specialist), after careful questioning of your symptoms, and a check for subtle body changes, may request special tests to diagnose if you have a problem with any part of your endocrine system.
Illustrations © Kathryn Skelsey (HealthArt Illustration)
Reviewed: March 2013