21. The Thryoid Gland Flashcards
Where is the thyroid gland?
Lies against and around front larynx and trachea
Below thyroid cartilage (Adams apple)
What are the 2 lobes of the thyroid joined by?
Isthmus
How does the thyroid gland develop?
First endocrine gland to develop
Appears as an epithelial proliferation in floor of pharynx at base of tongue
First descends as divertculum through thryoglossal duct
What is the histology of thyroid tissue?
Follicular cells arranged in spheres called thyroid follicles
Follicles filled with colloid, a deposit of thyroglobulin
Colloid is extracellular even though it is inside the follicle
What is the general structure of thyroid hormones?
2 tyrosine linked together with iodine at 3 or 4 positions on the aromatic rings
MIT + DIT = T3
DIT + DIT = T4 (thyroxine)
What is the role of thyroglobulin?
Acts as a scaffold on which thyroid hormones are formed
What is thyroid peroxidase?
Membrane bound enzyme that regulates 3 separate reactions involving iodide
What are the reactions that thyroid peroxidase is involved in?
- Oxidation of iodide to iodine
- Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin
- Coupling of MIT or DIT to generate thyroid hormones within the thyroglobulin protein
What has to happen to dietary iodine in the body?
Iodine reduced to iodide before absorption principally in the small intestine
What are the only molecules in the body that contain iodine?
Thyroid hormones and precursors
What are the dietary sources of iodine?
Iodised salt
Grains
Dairy products
Where is most T4 converted to T3?
Liver and kidneys
How are T3 and T4 transported in the blood?
Bound to the protein thyroxine-binding globulin
How is thyroid hormone regulated?
Via negative feedback
Describe the structure of the thyroid stimulating hormone
Glycoprotein hormone composed to 2 non-covalently bound subunits (alpha and beta)
The alpha subunit is also present in FSH and LH
Beta subunit provides unique biological activity
What does TSH stimulate?
Iodide uptake Iodide oxidation Thyroglobulin synthesis Thyroglobulin iodisation Colloid pinocytosis into cell Proteolysis of thyroglobulin Cell metabolism and growth
What happens when a TSH receptor couples to G alpha s subunit?
Adenylyl cyclase activated
Activates second messenger cAMP
Then activates protein kinase A
Leads to stimulation of thyroid hormone synthesis and release
What happens when a TSH receptor couples to a G alpha q subunit?
Activates phospholipase C
Activates second messengers DAG and IP3
Causes Ca2+ release via IP3 receptor and PKC activation
Leads to stimulation of thyroid hormone synthesis and release
What are the 3 general action of the thyroid hormone?
Increase in basal metabolic rate and heat production
Stimulation of metabolic pathways
Sympathomimetics effects
How doe thyroid hormones stimulate the metabolic rate in most tissues?
Increasing number and size of mitochondria
Stimulating the synthesis of enzymes in the respiratory chain
Which metabolic pathways are stimulated by thyroid hormones?
Lipid metabolism: stimulates lipolysis and betaoxidation of fatty acids
Carbohydrate metabolism: stimulate insulin dependent entry of glucose into cells and increase gluconeogenesis and glycogenolysis
What are the sympathetic effects of the thyroid hormone?
Increases target cell response to catecholamines by increasing receptor number on target cells
What are the effects of the thyroid hormone on the cardiovascular system?
Increases hearts responsiveness to catecholamines
Increased cardiac output
Increased peripheral vasodilation to carry extra heat to body surface
What are the effects of the thyroid hormone on the nervous system?
Essential for both development and adult function
Increased myelination of nerves and development of neurons
