The thyroid Flashcards
What is the thyroid? Where is it located, organised?
Thyroid gland is an organ located on top of to the trachea, superior to sternum and inferior to adams apple
Composed of 2 lobes in a butterfly shape either side of the trachea, connected by the isthmus
The cells are ogranised in follicular cells centered around a colloid, surrounded by parafollicular
What hormones are produced by the thyroid, and whats stimulates them?
TRH from the hypothalamus goes to adeohypophysis. Thyrotrophs produce TSH, which in turn activate TSHReceptor on Follicule cells of the thyroid
In response, Thyroid produces T3 and T4
What effects does TSH have on follicular cells? How does that lead to T3/4 production
The TSHR is linked to all processes of aking T3/4
1st action in increasing Iodine uptake from the blood, into the cell and into the colloid
It also helps produce Thyroglobulin protein and export it to colloid
TSHR also incease TPO and H2O2 at colloid cell membrane
All together, in colloid, Iondine is activated to the TG-either once (MonoIT) or twice (DiIT). These are then added together “coupled” => MIT+DIT= T3 and DIT+DIT=T4
CAREFUL-THE TG PART IS REMOVED-IT IS 2 TYROSINE RESIDUES TOGETHER
These are uptaken again by the follicule cells and sent to the blood
Which amino acid plays the most important role in TG?
Tyrosin residue in the protein recieves one or both of the Iodine, and when MIT and DIT are linked, its literally all thats left-
Tri Iodothyronine T3 or Tetra Iodothryronine T4
How are T3 and T4 transported in blood?
Most are bound to plasma proteins
70/80% bound to thyroid bindin prot, 10/15 to albumin, some to prealbumin
only about 0.05% of T4 and 0.5% of T3 are active and unbound
What is deiodination, and how does it relate to thyroid hormones?
T4 is the main produced thyroid hormone
Can be deiodinated to T3-active form IN TARGET tissue
IF deidonised at different/wrong position => Reverse T3, innactive
Summarise T3/T4 action on the body, the mechanism of action and their approximate lifespan
In cells T3 (T4 gets made to T3) mostly bind THR in nucleus and increase specific transcription of genes
Also has an effect on ion channels and mitochondria, stimilating them
The main actions are -Early foetal growth and development (if not there early-cretinism)
In adults-makes EVERYTHING GO FAST
Increase basal metabolic rate, protein cardo and fat metabolism, increase heart rate, potentiates Catecholamines (heart rate, lipolysis), affect GI, CNS, Gonads
ALL GOES FASTER
T3 lives about 2 days, and T4 around a week
How are T3/T4 regulated in regads of negative feedback and other?
TRH from hypothalamus to adenohypophysis Thyrotrophs making TSH, to thyroid making T3/4
BUT T4/3 direct negative feedback to Thryotrophs, and indirect to hypothallamus
ALSO-oestrogens increase TSH, Glucocorticoids and somatostatin DECREASE TSH
Iodine also decreases Thyroid producion of T3/4
