Glands and hormones
The endocrine system is composed of a series of hormonal organs and glands that produce and secrete hormones that the body uses for a wide range of functions. They control many different bodily functions, including breathing, metabolism, reproduction, movement, sexual development and growth.
Hormones are chemicals produced by specific organs and glands and are sent through the bloodstream to the various tissues in the body. They give the tissues signals telling them what they are supposed to do. When the glands do not produce the right amount of hormones, diseases develop that can affect many aspects of life.
Hormone production is usually controlled by a kind of “thermostat”. If there is too little of a given hormone in the blood, the producing gland will be stimulated (usually by another hormone) to make more. If hormone levels are too high, stimulation will decrease. This can be compared to the heating in our homes. If it is too cold, the thermostat will turn the heat on and if the temperature is just right, the heat will turn off.
The stimulating hormone is in turn often controlled by another hormone (or hormones). This creates a network of hormones that we refer to as a “hormone axis”, such as the “thyroid axis” or the “adrenal axis”.
Pituitary gland or hypophysis
The pituitary gland or hypophysis is the master gland of the body, as it produces several hormones that manage processes or stimulate other glands to produce other hormones.
It is a small gland, about the size of a large pea, and it hangs off the brain on a stem.
The pituitary gland is itself controlled by the part of the brain just above it: the hypothalamus. Nerve cells in the hypothalamus produce hormones and neurotransmitters that reach the anterior lobe (front part) of the pituitary gland through the small blood vessels in the stem, where they stimulate or inhibit (slow down) the production of hormones. For example, growth hormone production in the pituitary gland is stimulated by growth hormone releasing hormone (GHRH) and inhibited by somatostatin, both of which are produced in the hypothalamus.
In addition, the pituitary gland also constantly measures the level of hormones that it stimulates. For example, if there is enough thyroid hormone in the blood, the pituitary gland will stop stimulating the thyroid gland to produce more thyroid hormone. When the concentration of thyroid hormones falls in the blood, the pituitary gland produces more thyroid stimulating hormone (TSH) again. This is called “positive or negative feedback”.
The situation is slightly different for the posterior pituitary lobe: the outgrowths of some nerve cells in the hypothalamus extend into the posterior pituitary, where they release hormones into the bloodstream.
The following hormones are released from the anterior part of the pituitary gland:
GH = Growth Hormone: stimulates growth of bones and muscles, among other things
PRL = Prolactin: stimulates production of breast milk
TSH = Thyroid Stimulating Hormone: spurs the thyroid gland to produce thyroid hormone
ACTH = Adreno Corticotropic Hormone: stimulates the adrenal gland to make cortisol
LH = Luteinizing Hormone: stimulates hormone production in the testes or ovaries
FSH = Follicle-Stimulating Hormone: stimulates hormone production and maturation of sperm or egg cells in the testes or ovaries, respectively
The following hormones are released from the posterior part of the pituitary gland:
ADH = Antidiuretic Hormone: reduces water release by the kidneys
Oxytocin: Contracts the uterus during labour and helps release breast milk from the breast. It also stimulates social interaction.
Thyroid gland
The thyroid gland is a small gland found in the neck below the Adam’s apple. It makes and releases thyroid hormones to help regulate body growth and metabolism.
The main hormone produced by the thyroid gland is thyroxine. This hormone controls the amount of energy used by the body to maintain vital processes such as breathing, circulation and digestion. Too much thyroxine makes the body work too fast, whereas too little makes the body slow down. The thyroid hormones also affect brain growth and metabolism in babies in the womb and up to the age of about two years.
Hypothalamus
The hypothalamus is a small region in the brain. It controls pituitary hormones by releasing the following types of hormones:
- Thyrotrophic-Releasing Hormone (TRH)
- Growth Hormone-Releasing Hormone (GHRH)
- Corticotrophin-Releasing Hormone (CRH)
- Gonadotropin-Releasing Hormone (GnRH)
These hormones regulate body temperature, appetite and weight, mood, sex drive, sleep and thirst.
Adrenal gland
The adrenal glands are located at the top of each kidney. They produce hormones, androgens and cortisol, which are released directly into the bloodstream. They help regulate metabolism, the immune system, blood pressure, response to stress and other essential functions.
Ovaries
The ovaries are part of the female reproductive system. They produce eggs and hormones, including oestrogen, testosterone and progesterone. These are vital to reproductive organ development, breast development, bone health, pregnancy and fertility.
Pancreas
The pancreas is a large gland located behind the stomach.
- It makes digestive fluids, which contain powerful enzymes. They are released into the small intestine after meals to break down and digest food.
- It maintains healthy blood sugar levels. It produces insulin, glucagon and other hormones.
Parathyroid glands
The parathyroid glands are a group of four small glands located behind the thyroid gland. They produce a hormone called parathyroid hormone, which is essential to proper bone development, because it helps control both calcium and phosphorous levels in the body.
Pineal gland
The pineal gland is situated in the middle of the brain. This gland releases melatonin, which helps the brain enter into sleep mode.
Testes
The testes are part of the male reproductive system and play an important role in male development. The testes produce sperm and androgenic hormones, including testosterone. Testosterone and its metabolites promote the growth of the penis, growth of facial and body hair, deepening of the voice and a growth spurt during puberty.
Other functions of testosterone include:
- Maintaining sex drive
- Stimulating production of sperm
- Maintaining healthy levels of muscle and bone mass
Normal Puberty
In girls, the first sign of puberty is the breast development, influenced by female hormones. This usually starts between the age of 8 and 13 years. There is a large variance in age at start of puberty. On average, 2-3 years later, menarche occurs. After the first signs of puberty, the height velocity increases. After menarche, a girl still grows 6 to 7 cm. In boys, puberty start a little later, between the age of 9 and 14 years, when the testicular volume becomes 4 ml or more. It take 6 to 9 months after the onset of puberty before growth acceleration occurs.
