Thyroid Physiology Flashcards
When does the thyroid develop during embryogenesis?
4th week
Where does the thyroid originate from?
Tongue - endodermal thickening occurs forming the foramen caecum
How do C cells become part of the thyroid?
As it comes in contact with the lower pharynx
What is the final location of the thyroid gland, when does it settle here?
C5-T1 - by the 8th week
What happens to the thyroglossal duct?
Most disintegrate
What can happen as the thyroid migrates down?
Thyroglossal cysts can form
How heavy is the thyroid gland?
Around 25g
What chemical is required in order for the gland to secrete hormones?
Iodine
What is the gland made up of?
Follicles surrounded by parafolicular cells
Name the chemical produced by parafollicular cells
calcitonin
Describe the pathway leading to thyroid hormone synthesis
- Follicular cells produce thyroglobulin (tyrosine rich protein)
- Iodide ions are actively transported into the follicular cell where they are concentrated
- Enzyme thyroperoxidase converts iodide to iodine
- Iodine & thyroglobulin move into the colloid
- Iodine attaches to tyrosin residue
Name the product if one iodine attaches
Monoiodotyrosin (MIT)
Name the product if two iodines attach
Di-iodotyrosin (DIT)
What are the two potential products from MIT and DIT?
MIT + DIT = triiodothyronine (T3)
DIT + DIT = thyroxine (T4)
Which is more potent T3 or T4?
T3 - biologically active hormone
How does the body signal to the thyroid gland to produce T3 & T4?
Hypothalamus releases TRH stimulating the pituitary which produces TSH that binds to thyroid leading to the production of T3 & T4
How are thyroid hormones transported in the blood?
Bound to serum proteins or free in plasma
Name three serum proteins thyroid hormones bind to
- thyroxine binding globulin
- thyroxine binding pre-albumin
- albumin
In what form can thyroid hormones act on organs/tissues
Free in the plasma
What effect does thyroid hormone have on the liver?
Increases glycogenolysis & gluconeogenesis
What effect does thyroid hormone have on adipose tissue?
increased lipolysis
What effect does thyroid hormone have on the lungs?
increased breathing rate
What effect does thyroid hormone have on the heart?
increased heart rate & force of contraction
Overall what is the effect of thyroid?
Increases metabolic rate
How does thyroid increase metabolic rate?
- increase number/size of mitochondria
- increase oxygen use & ATP hydrolysis
- increase synthesis of enzymes
How do thyroid hormones increase sympathomimetic action?
Increase sympathetic response by increasing number of receptors
Name three types of de-iodinases & where are they found
D1 (liver & kidneys)
D2 (heart, muscle, CNS, fat, thyroid, pituitary)
D3 (foetal tissue & placenta, brain)
What do D1 & D2 do?
Convert T4 - T3
Describe the action of D3
Converts T3 into inactive T3 which is rapidly excreted
