Thyroid Gland Flashcards
Where is the thyroid gland?
in the neck on the anterior surface of the trachea immediately below the larynx
What is the thyroid gland made up of?
2 lobes connected by the isthmus
What do follicle cells do?
synthesise Tg and secrete it into the colloid of the thyroid follicles
What are the functional units for TH production?
thyroid follicles
What 2 cell types make up thyroid follicles?
follicular and colloid cells
What is the colloid?
the extracellular space where Tg with attached iodine atoms are stored
Where are C cells/parafollicular cells and what do they do?
in between the follicles and secrete calcitonin
What are the 2 basic ingredients of TH?
tyrosine and iodine
When is iodine reduced to iodide?
prior to absorption by the small intestine
Where do most steps of TH synthesis occur?
on Tg molecules within the colloid
What is Tg?
a protein made in the ER that is the first basic unit in which TH is made
What is the backbone and teeth of the TH ‘comb’?
- backbone = Tg
- teeth = tyrosine amino acids
What happens to iodine from the diet?
it is soaked up into the thyroid gland
What are the 5 basic steps of TH synthesis?
- iodide trapping
- iodination
- coupling
- colloid resorption
- Tg proteolysis
What are the 10 steps of TH synthesis?
- tyrosine containing Tg is exported from the follicular cells into the colloid via exocytosis
- thyroid captures I- from the blood and transfers it into the colloid by an iodide pump then the Na+/K+/I- antiport pump transports Na+ and I- the follicular cell
- iodide is oxidised to active iodide by TPO then exits through a channel into the colloid
- membrane bound TPO attaches iodide to a tyrosine within the Tg molecule
- T3 or T4 are produced and both products remain attached to Tg by peptide bonds
- stimulation for hormone secretion causes the follicular cells to internalise a portion of the Tg-hormone complex by phagocytosing a piece of colloid
- lysosomes attack the engulfed vesicle and split the iodinated products from Tg
- T3 and T4 diffuse freely through the outer membranes of the follicular cells into the blood
- Iodinase removes the iodide from MIT and DIT allowing the free iodide to be recycled for more synthesis
- TH molecules in the blood bind very quickly to plasma proteins for storage
How is MIT and DIT produced respectively?
- MIT = attachment of 1 iodine to tyrosine within the Tg molecule
- DIT = attachment of 2 iodines to tyrosine within the Tg molecule
What are THs bound to for storage in the plasma and colloid respectively?
- plasma = plasma proteins
- colloid = Tg
Which proteins have the largest and smallest capacity to bind T4 respectively?
- largest = albumin
- smallest = thyroxine-binding globulin (TBG)
Which TH is more free and biologically active?
T3 (10x more)
What is T4 metabolised by?
conjugation with glucuronic acid in the liver
What can T4 be used for?
a reserve i.e. only an enzyme is needed to break it down from Tg rather than having to synthesise from scratch
What is the main negative feedback pathway used to control the levels of TH?
the HPT (hypothalamus pituitary thyroid) axis