Insulin:

• accelerates the entry of glucose into the cells
• glucose is converted to glycogen (glycogenesis) in the liver
• glucose is converted to glycogen (glycogenesis), as well as activating the mTOR pathway to convert amino acids to proteins, in the muscles

When the glucose concentration in the blood decreases beyond the physiological limits, there is an immediate decrease in insulin secretion, and an increase in the secretion of glucagon from the alpha-cells of the pancreas. Glucagon causes:

• hydrolysis of glycogen (glycogenolysis) in the liver and promotion of the glucose into the blood
• breakdown of fat in the subcutaneous tissue into fatty acids, which are also promoted into the blood

In the end, insulin lowers blood glucose levels, and glucagon increases blood glucose levels.

There are 2 main hormonal disorders associated with the pancreas:

• Type I diabetes mellitus: lack of sufficient insulin in the blood, due to reduced production by the β-cells of the pancreas. This type appears suddenly during childhood (juvenile diabetes), due to the destruction of pancreatic β-cells by the cells of the immune system (autoimmune disease).
• Type II diabetes mellitus: reduced insulin production or the inability of the target cells to recognize insulin molecules, due to changes in the protein receptors of their cell membrane. This type is more common in adults 45 and older. This type is related to genetic predisposition and obesity.
• Diabetes mellitus is NOT the same as diabetes insipidus, a condition resulting in excessive thirst and issues with the chemical antidiuretic hormone (ADH) which regulates water reabsorption.

Diabetes mellitus causes:

• hyperglycemia, increased amounts of glucose in the blood
• glycosuria, increased amounts of glucose in the urine
• polyuria, large amounts of glucose that must be excreted
• polydipsia, large loss of water in the urine
• polyphagia, fact that cells are not supplied with glucose, leading to an insatiable sense of hunger
• weight loss, because of continuous breakdown of fats and proteins by the cells
• bad breath, due to acetone produced during the continuous breakdown of fat by the cells
• (in severe cases) decrease in the pH of the blood, due to the acidity of the byproducts from the catabolism of fat

There is no cure. Type I diabetes is treated with IV administration of insulin to the patient. Type II is treated with physical exercise, a special diet and antidiabetic medication.

Hypersecretion of insulin into the blood results in decreased concentration of glucose in the blood which can lead to coma.

Somatostatin inhibits the release of numerous hormones from the pituitary gland, pancreas, and gastrointestinal tract.

Ghrelin is also known as the hunger hormone, because it stimulates appetite and thus increases food intake. It also increases growth hormone and fat deposition. It's mainly secreted by the stomach.

Pancreatic Polypeptide (PP) slows down gastric emptying, relaxes gallbladder and reduces bile secretion, reduces the secretion of pancreatic digestive enzymes in the small intestine.

Adrenal Glands

Two small endocrine glands on the upper surface of the kidneys. Each adrenal gland consists of two distinct parts: the medulla (inner) and the cortex (outer).

The adrenal medulla: In cases of fear, anger, stress or excitement, the adrenal medulla receives nerve stimuli from the sympathetic nervous system and secretes adrenaline and noradrenaline. Their secretion into the blood causes similar effects as the sympathetic nervous system:

• increase in the rate of glycogenolysis (glycogen → glucose) in the liver
• breakdown of fats, release of fatty acids into the blood
• heart rate increase
• widening of the bronchi and bronchioles and acceleration of breathing rate
• constriction of blood vessels of the skin, to direct more blood to the muscles, leading to pale skin
• slowing down of the function of smooth muscles (for example in the digestive system and the kidneys) to direct more blood to the skeletal muscles and the brain, which is why we feel nausea or “butterflies”
• erection of the hairs of the skin, due to the stimulation of the muscles of their roots, to appear larger and more dangerous

All these reactions are part of the "fight or flight" mechanisms, which is closely related to the sympathetic nervous system. In contrast, the parasympathetic nervous system is responsible for the “rest and digest” mechanism, which relaxes the body to conserve energy and increases the function of the digestive system.

The adrenal cortex: stimulated by stress (such as illnesses), but mainly due to the adrenocorticotropic hormone (ACTH) rather than nervous signals. The secretory cells of the cortex then secrete corticosteroids. Corticosteroids are distinguished into three groups according to their action:

• Glucocorticoids (cortisol):
• • stimulate anti-inflammatory mechanisms and inhibit the immune system
  • stimulate gluconeogenesis (muscle proteins → amino acids → glucose in the liver)
  • promotes the use of fats for energy
  • increase the synthesis of glycogen in the liver from the excess glucose produced by gluconeogenesis and from the breakdown of fats
Mineralocorticoids (aldosterone):
• increases the reabsorption of sodium and chloride (NaCl) ions and the excretion of potassium ions (K)
• promotes the reabsorption of water in the kidneys from the filtrate thus increasing in arterial pressure
• Sex corticosteriods:
  development of secondary sex characteristics

Addison's disease. Associated with hypofunction of the adrenal cortex (less Aldosterone, Cortisol, Testosterone, Estradiol, Progesterone etc.):

• hypotension, due to increased excretion of Na+, Cl- and water with the urine.
• increased amounts of K+ ions in the blood and cells, due to their reduced excretion by the kidneys (kidneys under the influence of aldosterone use the Na/K pump to swap Na+ from the filtrate with K- from the blood, as well as Cl- pumps to take ions from the filtrate to the blood)
• reduced ability to cope with stress and anxiety, due to the reduced cortisol
• intense fatigue, due to reduced concentration of glucose in the blood
• hair loss, due to the reduced secretion of sex corticosteroids
• (sometimes) dark skin, due to increased production of ACTH and therefore MSH

Addison's disease is treated with the administration of corticosteroids and a diet rich in NaCl and poor in K.

Cushing's disease. Associated with hyperfunction of the adrenal cortex (more Aldosterone, Cortisol, Testosterone, Estradiol, Progesterone etc.):

• thin limbs, due to proteins being broken down into amino acids that are converted to glucose.
• fat body and face, due to high amount of glucose from gluconeogenesis.
• diabetes mellitus, due to high amount of glucose in the blood.
• hypertension, due to large amounts of NaCl and water being retained.
• excessive hair growth, due to excessive secretion of sex corticosteroids.

Cushing's disease is treated with surgical intervention, radiation or medication.

Thymus

Located in the thoracic cavity. Starts developing from the moment of birth until puberty, and then it shrinks and atrophies. It supplies the lymph nodes with T-cells, which have differentiated from simple lymphocytes within the gland. The thymus secretes thymosin during childhood, which causes the lymphocytes to mature and differentiate into active T-cells. It is responsible for “training” T-cells to detect foreign or cancer cells and distinguish them from healthy tissue, preventing autoimmune diseases.

Pinal Gland

Small tissue near the center of the brain, which secretes the hormone melatonin, a derivative of tryptophan. Serotonin is also a derivative of tryptophan that is converted into melatonin. It regulates the sleep-wake cycle and circadian rhythm. Melatonin levels rise when dark and drop when bright. Additionally, melatonin levels peak during early childhood and start declining during puberty.

SUMMARY

Written by Alexandros Samoutis

Endocrine Gland	Associated Hormnones	Chemical Class	Effects
Pituitary (anterior)	Growth Hormone (GH)	Protein	Promotes growth of body tissues
Pituitary (anterior)	Thyroid Stimulating Hormone (TSH)	Glycoprotein	Stimulates thyroid
Pituitary (anterior)	Adrenocorticotropic hormone (ACTH)	Peptide	Stimulates hormone release by adrenal cortex
Pituitary (anterior)	Follicle Stimulating Hormone (FSH)	Glycoprotein	Stimulates gamete production
Pituitary (anterior)	Leutenizing Hormone (LH)	Glycoprotein	(In men) Stimulates androgen production by gonads. (In women) Triggers ovulation and stimulates the production of estrogens and progesterone.
Pituitary (posterior)	Antidiuretic Hormone (ADH)	Peptide	Stimulates water reabsorption by kidneys → Decreases mean plasma osmolarity.
Pituitary (posterior)	Oxytocin	Peptide	Stimulates uterine contractions during childbirth
Thyroid	Thyroxine (T4), triiodothyronine (T3)	Amine	Stimulate basal metabolic rate
Thyroid	Calcitonin	Peptide	Reduces blood Ca2+ levels
Parathyroid	Parathyroid Hormone (PTH)	Peptide	Increases blood Ca2+ levels
Adrenal (cortex)	Aldosterone	Steroid	Increases blood Na+ levels → Increases mean hydrostatic blood pressure
Adrenal (cortex)	Cortisol, corticosterone, cortisone	Steroid	Increase blood glucose levels → Helps manage stress; Suppresses the immune system → Helps reduce inflammation
Adrenal (medulla)	Epinephrine, norepinephrine	Amine	Stimulate fight-or-flight response
Pineal	Melatonin	Amine	Regulates sleep cycles
Pancreas	Insulin	Protein	Reduces blood glucose levels
Pancreas	Insulin	Protein	Increases blood glucose levels
Testes	Testosterone	Steroid	Stimulates development of male secondary sex characteristics and sperm production
Ovaries	Estrogens and progesterone	Steroid	Stimulate development of female secondary sex characteristics and prepare the body for childbirth