Lecture 8.1 - The Thyroid Gland PDF

Anatomy of the thyroid gland: ◦Can be easily palpated Histology of the thyroid gland: ◦Follicular cells can vary in height and shape (cuboidal or columnar when activated and squamous shape when deactivated) ◦Thyroglobulin = glycoprotein Hormones of thyroid gland: ◦Thyrogl...

Anatomy of the thyroid gland: ◦Can be easily palpated Histology of the thyroid gland: ◦Follicular cells can vary in height and shape (cuboidal or columnar when activated and squamous shape when deactivated) ◦Thyroglobulin = glycoprotein Hormones of thyroid gland: ◦Thyroglobulin serves as a scaffold ◦T4 acts as a prohormone for T3 (which is biologically active) Iodine balance: ◦Iodine is a trace element in soil and water - so can be found in seafood for example ◦Iodine deficiency is the world's greatest single cause of preventable brain damage and mental retardation ◦Dietary sources of iodine: ‣ Some fruit and vegetables - concentration of iodine in plant foods is determined by iodine levels in the soil ‣ Cereal and grains - concentration of iodine in plant foods is determined by the iodine levels in the soil ‣ Milk and dairy products ‣ Fish and shellfish ◦Regions at risk for iodine deficiency: ‣ Northern Indian subcontinent ‣ Andean region in South America ‣ Mountainous regions of Europe, China and Africa ◦Daily physiological requirement of iodine in adults in 150ug  Generation of thyroid hormones: ◦Iodide is absorbed from the diet ◦Organification means adding the iodine to an organic benzene ring Steps in thyroid hormone synthesis: ◦TSH is released by the pituitary gland Key points synthetic process thyroid hormones: ◦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: humans can ingest a diet completely devoid of iodide for up to 2 months before a decline in circulating thyroid hormone levels is seen  ◦Although T4 is the major secretory product of the thyroid gland, T3 is the most active form of the hormone ◦Functions of thyrocytes: ‣ Collect and transport iodine ‣ Synthesise thyroglobulin and secrete it into the colloid ‣ Fix iodine to the thyroglobulin to generate thyroid hormones ‣ Remove the thyroid hormones from thyroglobulin and secrete them into the circulation Thyronine-specific deiodinases: ◦Although T3 is more active than T4, almost all hormonal output of the thyroid gland is T4 ‣ This "problem" of secreting the less active form is solved by the peripheral conversion of T4 to T3 by target tissues, through the action of thyronine- specific deiodinases ◦Deiodinases (D) regulate: ‣ Conversion of T4 to T3 in peripheral tissues ‣ Conversion of T4 and T3 to inactive metabolites Wolff-Chaikoff effect: 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: ◦The thyrotropin-releasing hormone (TRH) stimulates the anterior pituitary gland cells to increase their output of TSH ◦TRH receptors activate phospholipase second messenger system Regulation of thyroid hormone secretion - TSH: ◦The thyrotropin-stimulating hormone (TSH) stimulates the thyroid follicular cells to secrete thyrotropines ◦TSH receptors activate cAMP second messenger system ◦Effects of TSH on thyroid gland:  ‣ Increased proteolysis of the thyroglobulin ‣ Increased rate of "iodide trapping" ‣ Increased iodination of tyrosine to form the thyroid hormones ‣ Increased size and secretory activity of the thyroid cells ‣ Increased number of thyroid cells plus a change from cuboidal to columnar cells Thyroid hormone-receptor binding: Physiological effects of thyroid: Cardiovascular and respiratory effects: Metabolic effects: Effects on growth and development: ◦Thyroid hormone is required for growth to adult stature ◦Thyroid hormones act synergistically with growth hormone and somatomedins to promote bone formation ◦Thyroid hormones promote ossification and fusion of bone plates and bone maturation ◦The progression of tooth development and eruption depends on thyroid hormone ◦Growth and maturation of the epidermis, its hair follicles and nails ◦Thyroid hormone is extremely is extremely important for normal neurological development ◦Infants, insufficient foetal thyroid hormone causes congenital hypothyroidism (cretinism) characterised by irreversible intellectual disability and short stature