Lecture 8.1 - The thyroid gland

Anatomy of the Thyroid Gland

• The thyroid gland can be easily palpated.

Histology of the Thyroid Gland

• Follicular cells can vary in height and shape, being cuboidal or columnar when activated and squamous when deactivated.
• Thyroglobulin is a glycoprotein.

Hormones of Thyroid Gland

• Thyroglobulin serves as a scaffold for thyroid hormone synthesis.
• T4 acts as a prohormone for T3, which is biologically active.

Iodine Balance

• Iodine is a trace element in soil and water, and can be found in seafood.
• Iodine deficiency is the world's greatest single cause of preventable brain damage and mental retardation.
• Dietary sources of iodine include:
  • Some fruit and vegetables, with concentration dependent on soil iodine levels.
  • Cereal and grains, with concentration dependent on soil iodine levels.
  • Milk and dairy products.
  • Fish and shellfish.
• Regions at risk for iodine deficiency include:
  • Northern Indian subcontinent.
  • Andean region in South America.
  • Mountainous regions of Europe, China, and Africa.
• The daily physiological requirement of iodine in adults is 150ug.

Generation of Thyroid Hormones

• Iodide is absorbed from the diet.
• Organification involves adding iodine to an organic benzene ring.

Steps in Thyroid Hormone Synthesis

• TSH is released by the pituitary gland.

Key Points of Thyroid Hormone Synthesis

• Thyroid hormones contain large amounts of iodine, which must be adequately supplied in the diet.
• Synthesis of thyroid hormones is partially intracellular and partially extracellular.
• Colloid is a reservoir of thyroid hormones, allowing humans to survive for up to 2 months without iodine in their diet.
• Although T4 is the major secretory product, T3 is the most active form of the hormone.
• Thyrocytes collect and transport iodine, synthesize thyroglobulin, fix iodine to thyroglobulin, and secrete thyroid hormones into the circulation.

Thyronine-Specific Deiodinases

• Almost all hormonal output of the thyroid gland is T4, which is converted to T3 in peripheral tissues through the action of thyronine-specific deiodinases.
• Deiodinases regulate the conversion of T4 to T3 in peripheral tissues and the conversion of T4 and T3 to inactive metabolites.

Regulation of Thyroid Hormone Secretion

• Specific feedback mechanisms operate through the hypothalamus and anterior pituitary gland to control the rate of thyroid secretion.
• Regulation of thyroid hormone secretion is under negative-feedback control.
• TRH and TSH are released at different times.

Regulation of Thyroid Hormone Secretion - TRH

• TRH stimulates the anterior pituitary gland cells to increase their output of TSH.
• TRH receptors activate the phospholipase second messenger system.

Regulation of Thyroid Hormone Secretion - TSH

• TSH stimulates the thyroid follicular cells to secrete thyrotropines.
• TSH receptors activate the cAMP second messenger system.
• Effects of TSH on the thyroid gland include:
  • Increased proteolysis of thyroglobulin.
  • Increased rate of "iodide trapping".
  • Increased iodination of tyrosine to form thyroid hormones.
  • Increased size and secretory activity of the thyroid cells.
  • Increased number of thyroid cells, with a change from cuboidal to columnar cells.