Lecture 21: Thyroid Hormones Flashcards
How many mg of iodine (iodides such as sodium iodide) are required each year?
50 mg
True or False:
Iodine is absorbed from the gut tract in the same manner as chlorides.
True
Is the sodium-Iodide symporter (2:1) located on the basal or apical membrane of the thyroid cell?
Basal membrane
Where does the energy to drive the sodium-iodide symporter come from?
sodium-potassium ATPase pump
How much more concentrated can iodide be inside the cell compared to its concentration in the plasma?
30-250x plasma
What is the chloride-iodide counter-transporter also known as?
pendrin
What transporter pumps iodide across the apical membrane of the thyroid cell into the follicle?
Chloride-Iodide counter-transporter
aka - Pendrin
What membrane bound enzyme catalyzes the conversion of iodide into iodine (I2)?
peroxidase
What enzyme catalyses the combing of iodide with thyrosine on thyroglobulin?
peroxidase
What does the combining of iodine with tyrosine on thyroglobulin result in?
- Monoiodotyrosine (MIT)
- Diiodotyrosine (DIT)
catalyzed by peroxidase - which is a membrane enzyme
What thyroid hormone is produced by the recombination of DIT + DIT?
thyrosxine (T4)
more T4 is formed than T3
What thyroid hormone is produced by the recombination of DIT + MIT?
triiodothyronine (T3)
more active than T4 - but more T4 is formed
What is attached to a thyroglobulin molecule when it is stored in the colloid until needed?
- MIT
- DIT
- T3
- T4
Study and know figure 77-2 well for the exam.
Iodine transport.
What does thyroid cells secrete into the follicle?
thyroglobulin (MW=335,000)
How many tyrosine amino acids does each thyroglobulin molecule have that binds iodine to form thyroid hormones?
70 tyrosine amino acids
What form are iodide ions converted to so that they can directly bind to tyrosine amino acids on thyroglobulin molecules to form thyroid hormones
Iodide ions are converted to an oxidized form of iodine to directly bind to tyrosine.
> nascent iodine (I2) or
I3-
What is the major hormonal product formed during iodine metabolism to produce thyroid hormones?
thyroxine (T4)
How many molecules of thyroxine does each thyroglobulin molecule contain?
30 molecules of thyroxine.
True or False:
The thyroid can store several months supply of thyroid hormones.
True
Go through the mechanism of the release of thyroid hormones.
> Apical surface of thyroid cells pinch off small portions of the follicular colloid to form pinocytic vesicles.
> Lysosomes fuse with vesicles and digest thyroglobulin molecules to release thyroxine and triiodothyronine (thyroid hormones).
> Thyroid hormones diffuse into blood.
> Most of the iodinated tyrosine in the thyroglobulin does not become thyroid hormones but is recycled using a deiodinase enzyme (deficiency mimics iodine deficiency).
> Most of released thyroid hormone is thyroxine which then is slowly deiodinated to form triiodothyronine.
> Conversion from T4 to T3 requires 5’-iodinase
> T3 is more active than T4.
> Most thyroid hormones entering blood are immediately bound to plasma proteins synthesized in the liver.
- thyroxine-binding globulin (TBG):
- produced by liver
- hepatic failure - decreased total thyroid hormone level but normal free hormone levels.
> Plasma proteins have a high affinity for thyroid hormones, so the hormones are released slowly to tissue cells.
What enzyme is required for the conversion of T4 to T3?
5’-iodinase
What happens to most of the iodinated tyrosine in the thyroglobulin molecule?
Most of the iodinated tyrosine in the thyroglobulin does not become thyroid hormones but is recycled using a deiodinase enzyme.
a deficiency of deiodinase enzyme mimics iodine deficiency
What are most thyroid hormones that enter the blood stream immediately bound to?
Plasma proteins (thyroxine-binding globulin (TBG)) that are synthesized in the liver.
plasma proteins have a high affinity for thyroid hormones, so the hormones are released slowly to tissue cells
