The Thyroid Gland - anatomy and function Flashcards
Where is the thyroid gland located?
Lies against and around the front of larynx and trachea
Below thyroid cartilage
Isthmus extends from 2nd to 3rd rings of trachea
What joins the 2 thyroid lobes?
The isthmus
Is the thyroid the first endocrine gland to develop?
Yes !
Describe the embryological development of the thyroid
At approx 3-4 weeks gestation, thyroid gland appears as an epithelial proliferation at base of tongue.
As the thyroid descends, it remains attached to the tongue via the thyroglossal duct. It passes infront of the hyoid bone on its journey.
The duct should subsequently degenerate
The thyroid tissue is made up of 2 cell types…
Follicular cells
Parafollicular cells
Describe a thyroid follicle
Thyroid follicles are comprised of follicular cells arranged in spheres with central colloid.
Colloid is extracellular even though it is inside the follicle
What do follicular cells produce?
Thyroid hormones
What do the parafollicular cells produce?
Calcitonin
What does the colloid store?
Thyroglobulin
What are thyroid hormones comprised of?
2 tyrosines linked together with iodine at 3 or 4 positions on the aromatic rings
T3 = triiodothyronine T4 = tetraiodothyronine (thyroxine)
What is T4 called?
Thyroxine
What does thyroglobulin act as?
A scaffold on which thyroid hormones are formed - they contain many tyrosines which are used to synthesise thyroid hormones
What is the role of thyroid peroxidase in thyroid hormone synthesis ?
Oxidation of iodide to iodine
Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin
Coupling MIT or DIT to generate thyroid hormones within the thyroglobulin protein
The thyroid gland contains what percentage of all iodine in the body?
90-95%
Dietary iodine is reduced to iodide before absorption principally in..
Small intestine
Iodide is taken up from blood by thyroid epithelial cells, which have what symporter?
Sodium- iodide symporter (iodine trap)
What are some common sources of dietary iodine?
Dairy products Grains Meat Vegetables Eggs Iodized salt
90% of thyroid hormone secreted is T4 or T3?
T4
Is the biological activity of T3 or T4 greater?
T3
T3 and 4 are transported in the blood bound to the protein…
Thyroxine binding globulin
If bound it is not biologically active
How is thyroid hormone secretion regulated?
Via negative feedback
Hypothalamus releases thyrotropin releasing hormone (TRH) , which acts on the anterior pituitary to release thyroid stimulating hormone (TSH).
TSH acts on the thyroid to increase release of thyroid hormones
Thyroid hormones effect virtually every cell in body. They have 2 broad interconnected responses…
Effect cellular differentiation and development
Effect metabolic pathways
TSH is what type of hormone?
Glycoprotein
In most tissues, thyroid hormones stimulate the metabolic rate by…
Increasing number and size of mitochondria
Stimulating the synthesis of enzymes in the respiratory chain
Do thyroid hormones generally stimulate catabolic or anabolic pathways?
Catabolic e.g lipolysis, beta oxidation of fatty acids
Also stimulates entry of glucose into cells, gluconeogenesis and glycogenolysis
What are the sympathomimetic effects of thyroid hormones?
Increase target cell response to catecholamines - by increasing receptor number on target cells
How do thyroid hormones effect CVS?
Increase heart’s responsiveness to catecholamines - increase CO
Increase in HR and force of contraction
Increase peripheral vasodilation to carry extra heat to body surface
How do thyroid hormones affect nervous system?
Increase myelination of nerves and development of neurons
What type of receptors are thyroid hormone receptors?
Nuclear receptors - act by modulating gene expression
When thyroid hormone enters nucleus of cell and binds to thyroid hormone receptor on DNA, what happens?
Binding relieves repression of gene transcription and the gene is now expressed.
Expression of new protein mediates the effects of thyroid hormone - triggers cellular response
What is the arterial supply to the thyroid from?
Superior and inferior thyroid arteries
What is the superior thyroid artery a branch of?
External carotid artery
What is the inferior thyroid artery a branch of?
Subclavian > thyrocervical trunk > inferior thyroid artery
What is venous drainage of the thyroid via?
Superior
Middle
Inferior thyroid vein
Once T4 enters the cell, what happens?
Mostly converted to T3
Thyroid hormones increase basal metabolic rate. What does this mean in simple terms?
The cell burns more energy
How do thyroid hormones affect the bones?
Increase resorption - thins out bones
