Endocrine 2

• Pituitary gland is a small gland known as the master gland of the endocrine system, it consists of the anterior pituitary and the posterior pituitary
• The hypothalamus is the master regulator of the pituitary gland, it is connected to the pituitary gland by the infundibulum
• The hypothalamus and the anterior pituitary are partially controlled by the hormone whose secretion they stimulate, this is defined as negative feedback or feedback inhibition
• positive feedback is rare in hormones since it causes deviations from homeostasis
• anterior pituitary produces 6 types of hormones

  • growth hormone
  • adrenocorticotrophic hormone
  • thyroid stimulating hormone
  • prolactin
  • leutinizing hormone
  • follicle stimulating hormone
• Growth hormone (somatotropin) is the most abundant anterior pituitary hormone. It promotes body growth by stimulating the production of insulin-like growth factor and it stimulates protein anabolism and fat metabolism
• Target cells of growth hormone are the liver, skeletal muscle and bones. It increases growth of bones, muscle and other tissues
• Growth hormone induces cells to use lipids as energy source, increasing the amount of carbohydrates in the blood
• Prolactin (lactogenic hormone) targets mammary glands and initiates milk secretion or lactation
• PRL functionning

  • during pregnancy, a high level of PRL promotes the development of the breasts in anticipation of milk secretion
  • at the birth of an infant PRL in the mother stimulates the mammary glands to begin milk secretion
• TSH promotes the growth and development of the thyroid, it can also cause the thyroid gland to secrete its hormones
• ACTH promotes growth and development of the cortex of the adrenal gland and stimulates the gland to synthesize and secrete some of its hormones
• FSH stimulates primary follicles in to grow toward maturity in females and stimulates sperm production in males
• LH stimulates the formation and ovulation of the corpus luteum in females to secrete progesterone and stimulates interstitial cells to secrete testosterone in males
• Negative feedback control

  • through negative feedback mechanisms, the hypothalamus adjusts the secretions of the anterior pituitary gland
  • the anterior pituitary adjusts the secretions of its target glands, which in turn adjust the activity of the target tissues
• The posterior pituitary (neurohypophysis) is found in the back, it is the smaller of the two lobes and does not produce hormones, its function is to store and release hormones made by the hypothalamus
• The posterior pituitary gland serves as a storage and release site for two hormones: antidiuretic hormone and oxytocin, the cells do not make the hormones (neurons of the hypothalamus make these hornones)
• Antidiuretic hormone functions

  • prevents the formation of a large volume of urine
  • helps the body conserve water and maintain water balance in the body
  • stimulates contraction of muscles in the walls of the small arteries, which increases blood pressure (ADH = vasopressin)
• Oxytocin functions

  • it stimulates contraction of uterine muscles
  • it causes milk ejection from the breasts of lactating women
  • **positive feedback mechanisms
• Disorders of the endocrine system generally involve hyposecretion (not enough hormone) and hypersecretion (too much hormone released), other times the disorder can be due to a faulty hormone receptor
• Gigantism is a condition of hypersecretion of GH by the anterior pituitary during the growth years, it results in an abnormal increase in length of bones
• Acromegaly is a condition of hypersecretion of GH by the anterior pituitary during adulthood, it does not increase the length of long bones but only bones of hands, feet, cheek and jaw increase in size + certain tissues such as the heart
• PRL disorders

  • hypersecretion of PRL may cause lactation in non-nursing women, disruption of the menstrual cycle and impotence in men
  • hyposecretion of prl usually insignificant except in women who want to nurse their children (milk production cannot be initiated or maintained)
• Hyposecretion of antidiuretic hormone can lead to diabetes insipidus, a condition in which the patient produces abnormally large amounts of urine
• The pineal gland is a tiny gland located on the dorsal aspect of the brain’s diencephalon region. It is a part of the nervous system as well as the endocrine system
• Pineal gland produces melatonin, which contributes to setting the body’s biological clock (circadian rhytym). Its secretion is inhibited by the presence of sunlight which can affect a person’s mood
• Pineal gland disorder is seasonal affective disorder, a type of depression that afflicts many people in the winter. Because daylight is shorter, more melatonin is produced which makes you more tired and more depressed
• The thyroid is located in the neck just below the larynx. It is composed of tiny structural units called follicles (site of thyroid hormone synthesis)
• The thyroid gland produces and secrete three hormones: thyroxine (T4) triiodothyronine (T3) and calcitonin. T4 contains four iodine atoms and T3 contains three iodine atoms.
• T3 and T4 characteristics

  • T3 is more potent than T4 and is considered the principal thyroid hormone
  • T4 binds more strongly to plasma globulins so is not removed from the blood as quick as T3
  • The small amount of T4 which enters tissues is converted to T3
  • T3 binds more efficiently than T4 to nuclear receptors in target cells which are found throughout the body
• T3 and T4 functions

  • increase metabolic rate
  • increase protein synthesis
  • stimulate growth
  • increase body’s demand for ATP
  • important regulator of body temperature
• Calcitonin controls calcium content of the blood by increasing bone formation by osteoblasts and inhibiting bone breakdown by osteoclasts. It works with parathyroid hormone to maintain calcium homeostasis
• Thyroid gland disorders:

  • Graves disease: hypersecretion of thyroid hormone
  • Crenitism: hyposecretion of thyroid hormone
  • Hyposecretion of thyroid hormone later in life can cause myxedema, decreased metabolic rate, weight gain, loss of hair and yellow dullness of the skin
• Parathyroid gland secretes parathyroid hormone (PTH), which is the main hormone the body uses to maintain calcium homeostasis. It acts on bone and kidney cells where calcium is reabsorbed from urine in the blood.
• Hyperparathyroidism causes increase in blood calcium levels, hypercalcemia and possible development of osteoporosis and kidney stones. Treatment is surgical removal of one of the four parathyroid glands
• Hypocalcemia is low levels of calcium which requires a lifetime of PTH replacement therapy