Session 10- The Thyroid Gland Flashcards
Where is the thyroid
Lies against and around the front larynx and trachea
Below thyroid cartilage
What joins the two lobes of the thyroid
Isthmus
When does the thyroid gland appear
3-4 gestation it appears as an epithelial proliferation in the floor of pharynx at the base of the tongue
What are follicles filled with
Colloid- a deposit of thyroglobulin
How are thyroid hormones formed
2 tyrosines linked together with iodine at three or 4 positions on the aromatic rings
3- T3
4- T4
What is T3
Triiodothyronine
What is T4
Tetraiodothyronine
What is the scaffold on which thyroid hormones are. Formed
Thyroglobulin
What is thyroid peroxidase
Membrane bound enzyme that regulates 3 separate reactions involving iodide
1) Oxidation of Iodide to Iodine (requires the presence of H2O2)
2) Addition of Iodine to tyrosine acceptor residues
on the protein throglobulin
3) Coupling of MIT or DIT to generate thyroid
hormones within the thyroglobulin protein
How is iodine absorbed
It is reduced to iodide before absorption principally in the small intestine by a sodium-iodide symporter
How is thyroid hormone secretion regulated
Negative feedback
What are the 2 interconnected responses of thyroid hormone
- effects on cellular differentiation and development
- effects on metabolic pathways
What is Thyroid stimulating hormone
Glycoprotein hormone composed of 2 non-covalently bound sub-units
Triggers the release of thyroid hormone
What does TSH stimulate
- Iodine uptake
- iodide oxidation
- thyroglobulin iodination
- Thyroglobulin synthesis
- Colloid pinocytosis into cell
- proteolysis of thyroglobulin
- cell metabolism and growth
General actions of thyroid hormone
- increase in basal metabolic rate and heat production.
- stimulation of metabolic pathway
- sympathomimetic effects
How does thyroid hormone increase basal metabolic rate and heat production
In most tissues (exceptions include brain, spleen and testis), thyroid hormones stimulate the metabolic rate by:
• Increasing the number and size of mitochondria
• Stimulating the synthesis of enzymes in the respiratory chain
How does thyroid hormone stimulate metabolic pathways
Catabolic pathways are generally stimulated more than anabolic.
• Lipid metabolism: Stimulates lipolysis and b-oxidation of fatty acids
• Carbohydrate metabolism: Stimulate insulin-dependent entry of glucose
into cells and increase gluconeogenesis and glycogenolysis
How does thyroid hormone have sympathomimetic effects
Increases target cell response to catecholamines by increasing receptor number in target cells
Thyroid hormone on CVS
Increase CO
Increase HR
Increase force of contraction
Increase peripheral vasodilation
Thyroid hormone on nervous system
Increase myelination of nerves and development of neurones
What type of receptor is the thyroid receptor
Nuclear receptor they bind DNA in the absence of hormone usually leading to transcriptional repression.
How do thyroid hormones bind fo intracellular receptor
It is lipid soluble and enters cell through thyroid hormone transporters
In the cell the thyroid hormones receptor is pre-bound to specif DNA sequences called hormone response elements- HRE in promoter region
Thyroid hormone enters nucleus and relives repression and now gene is now expressed
Expression of new protein
What is goitre
Enlargement of thyroid gland develops when thyroid gland is overstimulated
Accompanies hypo/hyper thyroidisme
What causes hypothyroidism
- failure of thyroid gland
- TSH or TRH deficiency
- Inadequate dietary supply of iodine
- Radioactive iodine
- Autoimmunity
- Post surgery
- congenital
- anti-thyroid drugs
