The Hypothalamus and Pituitary
The hypothalamus gland sits above the pituitary and stimulates it with various hormones. The pituitary gland comprises of an anterior and a posterior section, releasing separate hormones.
The anterior pituitary gland releases:
- Thyroid Stimulating Hormone (TSH)
- Adrenocorticotropic Hormone (ACTH)
- Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH)
- Growth Hormone (GH)
The posterior pituitary releases:
- Antidiuretic Hormone (ADH)
The Thyroid Axis
The hypothalamus releases thyrotropin-releasing hormone (TRH). This stimulates the anterior pituitary to release thyroid stimulating hormone (TSH). This in turn stimulates the thyroid gland to release triiodothyronine (T3) and thyroxine (T4).
T3 and T4 are sensed by the hypothalamus and anterior pituitary, and they suppress the release of TRH and TSH. This results in lower amounts of T3 and T4. In this way, the level of thyroid hormone is closely regulated to keep it within normal limits. This is called negative feedback.
The Adrenal Axis
Cortisol is secreted by the two adrenal glands, which sit above each kidney. The release of cortisol is controlled by the hypothalamus. Cortisol is released in pulses and in response to a stressful stimulus (it is a “stress hormone“). It has diurnal variation, which basically means that it is high and low at different times of the day. Typically cortisol peaks in the early morning, triggering us to wake up and get going, and is at it’s lowest late in the evening, prompting us to relax and fall asleep.
The hypothalamus releases corticotrophin release hormone (CRH). This stimulates the anterior pituitary to release adrenocorticotrophic hormone (ACTH). This in turn stimulates the adrenal gland to release cortisol.
The adrenal axis is also controlled by negative feedback. Cortisol is sensed by the hypothalamus and anterior pituitary, and it suppresses the release of CRH and ACTH. This results in lower amounts of cortisol. In this way, cortisol is closely regulated to keep it within normal limits.
Cortisol has several actions within the body:
- Inhibits the immune system
- Inhibits bone formation
- Raises blood glucose
- Increases metabolism
- Increases alertness
Growth Hormone Axis
Growth hormone releasing hormone (GHRH) is released from the hypothalamus. This stimulates growth hormone (GH) release from the anterior pituitary. Growth hormone stimulates the release of insulin-like growth factor 1 (IGF-1) from the liver.
Through this mechanism growth hormone works directly and indirectly on almost all cells of the body and has many functions. Most importantly growth hormone:
- Stimulates muscle growth
- Increases bone density and strength
- Stimulates cell regeneration and reproduction
- Stimulates growth of internal organs
Parathyroid Hormone (PTH) is released from the four parathyroid glands (situated in four corners of the thyroid gland) in response to low serum calcium. It is also released in response to low magnesium and high serum phosphate. It’s role is to increase the serum calcium concentration.
PTH increases the activity and number of osteoclasts in bone, causing reabsorption of calcium from the bone into the blood thereby increasing serum calcium concentration.
PTH also stimulates an increase in calcium reabsorption in the kidneys meaning that less calcium is excreted in the urine.
Additionally, it stimulates the kidneys to convert vitamin D3 into calcitriol, which is the active form of vitamin D that promotes calcium absorption from food in the small intestine.
These three effects of PTH (increased calcium absorption from bone, the kidneys and the small intestine) all help to raise the level of serum calcium. When serum calcium is high this suppresses the release of PTH (via negative feedback) helping to reduce the serum calcium level.
Renin is a hormone secreted by the juxtaglomerular cells that sit in the afferent (and some in the efferent) arterioles in the kidney. They sense the blood pressure in these vessels. They secrete more renin in response to low blood pressure and secrete less renin in response to high blood pressure. Renin is an enzyme that acts to convert angiotensinogen (released by the liver) into angiotensin I. Angiotensin I converts to angiotensin II in the lungs with the help of an enzyme called angiotensin-converting enzyme (ACE).
Angiotensin II acts on blood vessels to cause vasoconstriction. This results in an increase in blood pressure. Angiotensin II also stimulates the release of aldosterone from the adrenal glands.
Aldosterone is a mineralocorticoid steroid hormone. It acts on the nephrons in the kidneys to:
- Increase sodium reabsorption from the distal tubule
- Increase potassium secretion from the distal tubule
- Increase hydrogen secretion from the collecting ducts
When sodium is reabsorbed in the kidneys water follows it by osmosis. This leads to an increase in intravascular volume and subsequently blood pressure.
Last updated November 2018