Hormones are chemical messengers that control and coordinate a wide range of activities in the body. The regulation of hormone production, and their balance is fundamental to ensuring that the body works correctly. At the center of their complex regulation is the brain, and more specifically the hypothalamus and the pituitary gland.
Regulation of hormones
When talking about hormone level balance, it is important to understand that what the body is trying to achieve is to produce just the right amount of each hormone to fulfill the body’s needs in real-time. The ability to change certain hormone levels depending on the situation is necessary for survival. For example, when a person does not drink enough water, the body needs to find ways to save water to avoid dehydration. To do this, the pituitary gland secretes a hormone called vasopressin, which tells the kidneys to excrete less water, which results in the person’s urine being more concentrated.
One of the main systems that regulates the secretion of certain hormones is the hypothalamic-pituitary axis, which involves the exchange of hormonal signals between the hypothalamus and the pituitary gland in the brain, as well as the participation of the hormonal signals sent by other endocrine glands.
Through this axis, various body functions and hormone levels are regulated, including:
- Body growth.
- Milk production and lactation.
- Contraction of the uterus.
- Regulation of hydration and water control.
- Thyroid hormone levels.
- Estrogen and progesterone levels.
- Testosterone levels.
- Cortisol levels and stress response.
Pituitary gland regulation
The role of the hypothalamus in this case is to produce 7 different hormones, each of which travel to the pituitary gland to “deliver” a specific message: to produce more or less of its hormones.
The pituitary gland is in charge on the one hand of producing hormones that have a specific effect on certain tissues and organs, and on the other hand, producing hormones that stimulate the secretion of other hormones in the target endocrine glands.
The hypothalamus and pituitary gland themselves are also controlled by hormones and other signals from the body, which give them information about the body’s situation and makes them respond accordingly. How does this regulation work? When the target endocrine gland has produced enough hormone, the increased hormone level acts as a messenger, telling the hypothalamus and pituitary gland to stop stimulating its production. This is known as “negative feedback” and is used to regulate most of the hormone systems that involve the hypothalamic-pituitary axis.
Pituitary hormones and their functions
The pituitary gland produces over 8 different hormones to carry out its mission of regulating body functions and achieving adequate hormone balance.
- Adrenocorticotropic hormone (ACTH)
ACTH plays an important role in the body’s stress response. When a person experiences fear or a stressful situation, the hypothalamus releases a hormone called CRH, which triggers the release of ACTH by the pituitary gland. ACTH travels through the bloodstream to the adrenal glands, located on top of the kidneys, and causes them to release cortisol, also known as “the stress hormone”.
Cortisol then travels around the body, resulting in increased sugar levels, a higher respiratory rate, and an increase in blood pressure. Essentially getting the body ready to act, giving it a boost of energy, and making sure that it has enough oxygen, and that blood is pumped quickly to where it needs to go. This is what is commonly known as the “fight or flight response”.
In this case, the chain of events needs to be extremely fast to be able to react to danger, but once the danger is over, the body needs to go back to its normal state. The hypothalamus no longer receives the signal that there is danger, and the high levels of cortisol in the blood tell it to stop producing CRH. Once the pituitary gland stops receiving the signal from the hypothalamus, and senses the elevated levels of cortisol, it stops secreting ACTH, meaning that the adrenal glands are no longer stimulated to produce cortisol.
- Thyroid-stimulating hormone (TSH)
Thyroid hormones are produced, stored, and secreted by the thyroid gland. They are needed to regulate a person’s protein, carbohydrate, and fat metabolism, having an effect on practically all the different tissues of the body. The hypothalamus-pituitary axis is involved in regulating the levels of thyroid hormones.
When thyroid hormone levels are low, the hypothalamus produces higher amounts of a hormone called TRH, which stimulates the production of TSH by the pituitary gland. TSH then travels to the thyroid gland, promoting the production and release of thyroid hormones to the blood. This increase will continue until reaching the necessary hormone level, at which point the negative feedback process starts: the hypothalamus and the pituitary receive the signal to stop producing so much TRH and TSH.
Through this continuous process, the hormones are produced at the correct amount based on what the body needs at the time.
- Luteinizing hormone (LH) and follicle-stimulating hormone (FSH)
LH and FSH play a key role in the body’s sexual development and reproduction by stimulation the production of sex hormones.
The hypothalamus secretes a hormone called GnRH, which stimulates the production of LH and FSH by the pituitary gland. The production of GnRH is usually suppressed during childhood and is activated during puberty.
In women, these hormones regulate the menstrual cycle: FSH stimulates the growth and development of follicles in the ovary, as well as the production of estrogens, while LH triggers ovulation and stimulates the production of progesterone. Part of its regulation also depends on negative feedback, where the estrogens produced let the hypothalamus and pituitary gland know when to stop producing their hormones.
In men, FSH and LH act on the testes. LH stimulates the production of testosterone, and FSH together with testosterone ensures a normal sperm count and sperm quality is maintained.
- Prolactin (PL) and Oxytocin
Prolactin is a hormone that is produced by the pituitary gland and plays a key role in milk production and breast development. Oxytocin is also produced by the hypothalamus and is secreted by pituitary gland, and it is responsible for the emission of milk by the breast, or lactation. Together, these hormones regulate the process that allows a woman to breastfeed.
When a woman is not pregnant, the hypothalamus produces a molecule called dopamine, which prevents the production of PL in the pituitary gland.
During pregnancy, estrogen and progesterone levels are increased, which cause growth of the cells that produce PL in the pituitary gland, and of the breast tissue, all in preparation for the arrival of the baby.
The stimulation of the nipples by the baby suckling sends a signal to the hypothalamus to stop blocking the pituitary from producing prolactin. As a result, prolactin levels in blood increase, travelling to the breast tissue and triggering the production of milk. Suckling also signals the hypothalamus and pituitary to produce and secrete oxytocin, which travels to the breast, where it causes the milk to move into the milk ducts, allowing its ejection.
When the suckling stops, the hypothalamus begins to produce dopamine again, inhibiting the entire process of lactation. However, nipple stimulation is not the only factor that influences the production of PL; light, olfactory factors (smells) and stress also play a role.
When PL is not produced at the necessary amounts, issues can occur. Low concentrations of PL will result in a lack of milk production in lactating women, and elevated levels of PL can lead to the production of milk in women that are not breastfeeding, and even in men.
While Oxytocin plays a role in lactation, it is also involved in producing uterine contractions during labor. Altered levels of this hormone can also cause problems. For example, high levels during pregnancy results in abnormal growth of the uterus, making pregnancy difficult; while decreased levels of oxytocin slows uterine contractions during labor and reduced milk secretion.
- Vasopressin or antidiuretic hormone (ADH)
Vasopressin, also known as ADH, is a hormone that is produced by the hypothalamus but is secreted into the bloodstream by the pituitary gland. It plays a key part in controlling the body’s blood pressure, sodium, and water levels, by affecting the kidney’s ability to reabsorb water.
When the body has either too much sodium or too little water, the pituitary gland secretes ADH into the bloodstream, through which it travels to the kidneys. In response to this, the kidneys reabsorb more water, resulting in more concentrated urine.
In this case, ADH production is regulated by the changes in blood sodium and water levels, as well as blood volume. These changes are sensed by receptors in the hypothalamus, stimulating or inhibiting the production of ADH, depending on what the body needs.
- Growth hormone (GH) or somatotropin
So far, we have seen cases where the pituitary gland produces a hormone to stimulate other target glands to produce their hormones, and others where the pituitary glands hormone has a direct effect on certain tissues and organs. When it comes to the growth hormone produced by the pituitary, both situations occur.
The chain of events starts in the hypothalamus, with the production of GHRH, which stimulates the production of GH by the pituitary gland. GH acts directly on nearly every tissue, but especially the bone and cartilage, promoting their growth. It also stimulates the production of IGF-1 by cells in the liver, which then travels throughout the body, promoting growth and increasing the metabolism of many tissues.
The elevated level of the newly produced GH tells the hypothalamus and the pituitary gland to stop producing GHRH and GH. IGF-1 also helps reduce the levels of GH. It does so by telling the pituitary gland to stop producing GH, as well as promoting the production of somatostatin, a hormone that inhibits the production of GHRH and GH.
Overall, the production of GH is regulated by the hypothalamic-pituitary axis and other hormones such as somatostatin and ghrelin, but can be influenced by other factors, such as stress, exercise, nutrition, and sleep. Through its regulation system, GH is released in pulses instead of being kept at a constant level, meaning that the levels of GH vary hourly. The production of GH also varies throughout a person’s life, with high levels being produced during childhood, and maximum levels occurring during puberty, before slowly decreasing with age.
Research and history of hormone regulation
The pituitary gland was first described around the year 200 A.D. by Galen of Pergamon, and until the beginning of the 20th century, was thought to be responsible for removing nasal mucus from the brain. It’s true function began to be suspected when doctors and investigators noticed that patients suffering from certain diseases often had an enlarged pituitary gland, but it was not clear exactly how it was related.
Understanding the true importance of the pituitary gland and identifying how it works has only happened over the last 100 years. With the advancement of science and technology, it was possible to carry out research that enabled us to further understand the body’s hormonal system.
We now know that it is an incredibly complex and fine-tuned network involving different organ systems, glands, hormones, and even external factors. We are now able to diagnose and treat medical conditions and symptoms caused by their imbalance, but there is room for improvement. There is much we still do not know about the system.
Are there factors that influence hormone balance that we have not identified? Are there better treatment options that we have not discovered yet? Is there a better way to diagnose hormonal issues? Research is the only way to answer these questions.