The Endocrine System

Growth Hormone (GH)

• GH levels peak during adolescence, typically around puberty, as the body undergoes significant growth and development. This surge in GH levels is essential for the completion of physical growth and development.

• Infants and children have the highest levels of GH due to promotion of growth. In fact, GH is responsible for the rapid growth and development that occurs during childhood and adolescence. GH plays a crucial role in the growth of bones, muscles, and other tissues.

• GH release is regulated by two hypothalamic hormones: Growth Hormone-Releasing Hormone (GHRH) and Growth Hormone-Inhibiting Hormone (GHIH). GHRH stimulates the release of GH from the pituitary gland, while GHIH suppresses GH release, preventing excessive production.

Thyroid-Stimulating Hormone (TSH)

• Target tissue is the thyroid gland
• Stimulates normal development and secretory activity of the thyroid gland
• Release is triggered by Thyrotropin-Releasing Hormone (TRH) from the hypothalamus
• Rising blood levels of thyroid hormones inhibit TSH release

Adrenocorticotropic Hormone (ACTH)

• Stimulates the adrenal cortex to release corticosteroids
• Regulation of ACTH release is triggered by Corticotropin-Releasing Hormone (CRH) from the hypothalamus
• Internal and external factors such as fever, hypoglycemia, and stressors can alter CRH release

Gonadotropins (FSH and LH)

• FSH stimulates gamete production
• LH promotes production of gonadal hormones
• Regulation of gonadotropin release is triggered by Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus during and after puberty
• Absent from the blood in prepubertal boys and girls

Prolactin (PRL)

• Stimulates milk production
• Regulation of release is primarily controlled by Prolactin-Inhibiting Hormone (PIH) (dopamine)
• Decrease in PIH leads to increased PRL release
• PRL levels rise and fall in rhythm with estrogen levels

Thyroid Gland

• Located anterior to the trachea in the neck
• Releases thyroid hormones T3 and T4
• Gland composed of repeated follicles filled with thyroglobulin
• Thyroglobulin + iodine combines to form thyroid hormones which are then released into the bloodstream

Thyroid Hormone (TH)

• Major metabolic hormone that affects nearly every cell in the body
• Increases metabolic rate and heat production
• Regulates development and maintenance of tissues
• Dietary supply of iodine is needed for TH production

Calcitonin

• Released in response to high blood calcium ion levels
• Produced by parafollicular cells located in the thyroid gland
• Inhibits osteoclast activity and stimulates osteoblast activity
• Antagonist to Parathyroid Hormone (PTH)

Parathyroid Glands

• Four to eight tiny glands embedded in the posterior aspect of the thyroid gland
• Cells secrete Parathyroid Hormone (PTH)
• PTH - most important hormone in calcium homeostasis
• Secretes PTH at low calcium blood levels

Parathyroid Hormone (PTH)

• Stimulates osteoclasts to digest bone matrix and release calcium into the bloodstream
• Enhances reabsorption of calcium and secretion of phosphate by kidneys
• Promotes activation of vitamin D

Mineralocorticoids

• Regulates electrolytes (primarily sodium and potassium ions) in extracellular fluid
• Importance of sodium ions: affects extracellular fluid volume, blood volume, and blood pressure
• Importance of potassium ions: sets resting membrane potential of cells

Glucocorticoids

• Necessary for normal sperm production
• Maintains reproductive organs in a functional state

Glucocorticoid Drugs

• Used to control chronic inflammatory disorders due to their anti-inflammatory properties
• Decrease blood pressure by blocking renin and aldosterone secretion

Gonadocorticoids (Sex Hormones)

• Most weak androgens are converted to testosterone in tissue cells, while some are converted to oestrogen
• Contribute to:
  • Onset of puberty
  • Appearance of secondary sex characteristics
  • Sex drive in women
  • Oestrogen in postmenopausal women

Adrenal Gland

• Adrenal medulla secretes epinephrine and norepinephrine in response to short-term stress (fight or flight response)
• Adrenal cortex secretes cortisol and aldosterone in response to prolonged term stress

Pineal Gland

• Small gland located in the brain
• Secretes melatonin
• Melatonin affects:
  • Timing of sexual maturation and puberty
  • Sleeping patterns
  • Melatonin levels vary during the daily cycle

Hypothalamic Hormones

• Regulate growth, development, metabolism, and homeostasis
• All act on the anterior pituitary gland
• 4 releasing hormones and 2 inhibiting hormones

Hypothalamic-Pituitary Relationships

• Axons extend from the hypothalamus through the infundibulum (stalk of the pituitary gland) into the posterior pituitary lobe/gland
• GH levels peak during adolescence and decline with age
• Infants and children have the highest levels of GH due to promotion of growth needed in the individual

Growth Hormone (GH)

• Release is chiefly regulated by 2 hypothalamic hormones:
  • Growth hormone-releasing hormone (GHRH) stimulates the anterior pituitary to secrete GH
  • Growth hormone-inhibiting hormone (GHIH) inhibits the release of GH from the anterior pituitary

Thyroid-Stimulating Hormone (TSH)

• Target tissue is the thyroid gland
• Stimulates normal development and secretory activity of the thyroid gland (release of thyroid hormones)
• Release triggered by thyrotropin-releasing hormone (TRH) from the hypothalamus
• Rising blood levels of thyroid hormones inhibit TSH release

Adrenocorticotropic Hormone (ACTH)

• Stimulates the adrenal cortex to release corticosteroids
• Regulation of ACTH release:
  • Triggered by hypothalamic corticotropin-releasing hormone (CRH) in daily rhythm
  • Internal and external factors such as fever, hypoglycemia, and stressors can alter release of CRH

Gonadotropins

• Follicle-stimulating hormone (FSH) stimulates gamete (egg or sperm) production
• Luteinising hormone (LH) promotes production of gonadal hormones
• Regulation of gonadotropin release:
  • Triggered by gonadotropin-releasing hormone (GnRH) from the hypothalamus during and after puberty
  • Suppressed by gonadal hormones (feedback)

Prolactin (PRL)

• Stimulates milk production
• Regulation of release:
  • Primarily controlled by prolactin-inhibiting hormone (PIH) (dopamine)
  • Decrease in PIH leads to increased Prolactin release

Thyroid Gland

• Located anterior to the trachea in the neck
• Releases thyroid hormones T3 (triiodothyronine) and T4 (thyroxine)
• Gland composed of repeated follicles filled with the glycoprotein thyroglobulin
• Thyroglobulin + iodine combines to form the thyroid hormones which are then released into the bloodstream

Thyroid Hormone (TH)

• Major metabolic hormone which affects nearly every cell in the body
• Increases metabolic rate and heat production (calorigenic effect)
• Regulation of growth and development
• Maintenance of dietary iodine is required for production of T3 and T4

Endocrine System Across the Life Span

• Most endocrine organs operate well until old age
• Exposure to pollutants disrupts hormone function
• Sex hormones, thyroid hormone, and glucocorticoids are vulnerable to the effects of pollutants
• Interference with glucocorticoids may help explain high cancer rates in certain areas
• Ovaries undergo significant changes with age and become unresponsive to gonadotropins
• Testosterone also diminishes with age, but effect is not usually seen until very old age
• GH levels decline with age - accounts for muscle atrophy with age
• TH declines with age, contributing to lower basal metabolic rates
• PTH levels remain fairly constant with age, but lack of estrogen in older women makes them more vulnerable to bone-demineralising effects of PTH