Pituitary and Thyroid Glands Overview

Pituitary Gland and Thyroid Gland

• The pituitary gland (hypophysis) and hypothalamus work together to regulate many other endocrine glands.
• The hypothalamus's influence on hormone release differs between the anterior and posterior pituitary lobes.

Learning Outcomes

• Students should be able to describe the pituitary gland.
• Students should be able to describe the hormones released by the anterior and posterior pituitary gland.
• Students should be able to describe the hormones released by the thyroid gland and its regulation.

Anterior Pituitary Hormones

• Growth hormone (GH): Stimulates growth and division of body cells, especially in bones and skeletal muscles; regulates metabolism in organs like the liver, intestines, and pancreas; stimulates protein synthesis, promotes fat breakdown, and increases blood glucose levels. GH release is stimulated by growth hormone releasing hormone (GHRH) and suppressed by growth hormone inhibiting hormone (GHIH).
• Thyroid-stimulating hormone (TSH): Stimulates the synthesis and secretion of the thyroid hormones triiodothyronine (T3) and thyroxine (T4), both produced by the thyroid gland.
• Adrenocorticotropic hormone (ACTH): Controls the production and secretion of cortisol and other glucocorticoids by the cortex of the adrenal glands.
• Follicle-stimulating hormone (FSH): Stimulates the production of gametes (ova or spermatozoa) in both males and females. This plays a role in follicle development and estrogen production in women and sperm production in men.
• Luteinizing hormone (LH): In females, LH triggers ovulation and stimulates the formation of the corpus luteum; this corpus luteum secretes progesterone. Estrogen and progesterone prepare the uterus for implantation and prepare the mammary glands for milk production. In males, LH stimulates cells to secrete testosterone.
• Prolactin (PRL): Stimulates lactation, has a direct effect on the breasts immediately after childbirth. PRL release is stimulated by prolactin-releasing hormone (PRH). Conversely, it is reduced by prolactin-inhibiting hormone (PIH).
• Melanocyte-stimulating hormone (MSH): Increases skin pigmentation in amphibians. Its exact role in humans is unclear.

Regulation of GHRH & GHIH by Glucose

• A major regulator of GHRH and GHIH secretion is the blood glucose level.
• GHRH Mechanism:
  • Hypoglycemia stimulates the hypothalamus to secrete GHRH.
  • GHRH stimulates somatotrophs of the anterior pituitary to release human growth hormone.
  • Human growth hormone stimulates the secretion of insulin-like growth factors.
  • These factors speed up the breakdown of liver glycogen into glucose, increasing blood glucose levels to normal.
• GHIH Mechanism:
  • Hyperglycemia stimulates the hypothalamus to secrete GHIH.
  • GHIH inhibits the secretion of human growth hormone by somatotrophs.
  • Low levels of growth hormone slow down the breakdown of glycogen in the liver, and glucose is released into the blood more slowly.
  • Blood glucose levels fall to normal.

Factors Promoting Growth Hormone Secretion

• Decreased fatty acids
• Increased amino acids in the blood
• Deep sleep
• Increased activity of the sympathetic nervous system (stress or vigorous physical exercise); other hormones (glucagon, estrogens, cortisol, and insulin).

Factors Inhibiting Growth Hormone Secretion

• Increased levels of fatty acids
• Decreased levels of amino acids in the blood
• Rapid eye movement sleep
• Emotional deprivation
• Obesity
• Low levels of thyroid hormones
• Human growth hormone itself (through negative feedback)

Posterior Pituitary Hormones

• Oxytocin: Affects the mother's uterus and breasts during and after delivery. During labor, it enhances contractions of the uterus. After delivery, it stimulates milk ejection in response to stimuli such as a suckling infant. In males and nonpregnant females, its function isn't clear.
• Antidiuretic hormone (ADH or vasopressin): Decreases urine output by increasing the permeability of water and collecting tubules of the nephrons in the kidneys; this increases reabsorption of water. This hormone also decreases water loss through sweating and constricts arterioles, increasing blood pressure. Secretion of ADH is regulated by blood volume and osmotic pressure.

Other Factors Affecting Secretion

• Pain, stress, trauma, anxiety, acetylcholine, nicotine, drugs such as morphine and tranquilizers can stimulate ADH secretion.
• Alcohol inhibits ADH secretion, causing frequent urination.
• Hyposecretion of ADH or nonfunctional ADH receptors causes diabetes insipidus.