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
