Thyroid hormone physiology Flashcards
What cell synthesises hormones?
follicular cells
What is needed for the synthesis of T3 and T4?
iodine
Describe the synthesis of T3 and T4?
absorbed into the cell by iodine transporters (sodium-iodine symporter)
iodine attached to tyrosine residues on thyroglobulin to form MIT and DIT
What is the real name of MIT?
monoiodothyrosine
What is the real name of DIT?
di-iodothyrosine
What amounts of MIT and DIT make up T3?
MIT + DIT
What amounts of MIT and DIT make up T4?
DIT + DIT
Where is T3 and T4 stored before it is required?
colloid thyroglobulin
What percentage of hormone secreted is the highest - T3 or T4?
T4 = 90% T3 = 10%
Is T3 or T4 more potent?
T3
Are T3 and T4 hydrophobic or hydrophilic?
hydrophilic/lipophilic
How is T4 converted to T3?
by the liver
Which is more biologically active - T4 or T3?
T3
What do T3 and T4 bind to in blood?
plasma binding proteins e.g. thyroxine binding globulin, thyroxine binding prealbumin, albumin
What form of T3 and T4 is avaliable to tissues?
unbound form
How do the thyroid hormones affect sympathetics?
increase responsiveness to adrenaline and noradrenaline by increasing the number of receptors
What stimulates the release of T3 and T4?
thyroid stimulating hormone
How are T3 and T4 activated?
removing iodine
- occurs in liver and kidney
What is the role of deiodinase enzymes?
take part in the activation and deactivtion of thyroid hormones
Where are deiodinase enzymes found?
in liver and kidney heart and skeletal muscle CNS fat thyroid and pituitary foetal tissues, placenta
How does TSH bind to TRH?
on the surface of thyroid epithelial cells
What happens after TSH had bound to TRH?
G proteins are activated which convert GTP –> GDP and produce cAMP
cAMP increases the production and release of T3 and T4
What is the thyroid picture in primary hypothyroidism?
low T3&4
high TSH
What is the thyroid picture in primary hyperthyroidism?
high T3&4
low TSH
What is the thyroid picture in secondary hypothyroidism?
low T3&4
low/normal TSH
What is the thyroid picture in secondary hyperthyroidism?
high T3&4
high/normal TSH
What is the normal TSH values?
4mU/L
What is the normal free T4 values?
9.8-18.8 pmol/L
Why do goitres form?
reduced T3/T4 production causes a rise in TSH that causes a gland enlargment
