Thyroid hormone

Question 1

thyroid description:

Answer 1

butterfly-shaped endocrine gland, located at the front of the neck and
rests on the trachea, just below the larynx

Question 2

function of thyroid:

Answer 2

secretion of thyroid hormones
(it’s a vital organ)

Question 3

thyroid follicle:

Answer 3

o A follicular wall : formed of a single
layer of cubic epithelial cells
o A central cavity : containing a
substance called colloid or colloidal
substance

Question 4

Follicular cells description:

Answer 4

Follicular cells (vesicular cells or thyreocytes) :
 They are cube-shaped and are therefore oriented:
- Apical pole : on the side of the colloid
- Basal pole : on the side of the blood vessels
 They contain in their cytoplasm:
- a granular endoplasmic reticulum
- a Golgi apparatus
- numerous vesicles at the apical pole
They are therefore very active cells
due to their high protein synthesis activity
 They allow the exchange of molecules
between the blood and the colloid

Question 5

Biosynthesis of thyroid hormones:

Answer 5

• iodine = used w other components to produce thyroid hormones.
• provided by food
• The follicular cells capture iodides (I-) from the blood, by active transport (the
  intracellular iodine concentration is 30
  times higher than that of the blood)
• The iodides (I-) are then oxidized to active organic iodine (I2), at the top of the cell, following its oxidation by the enzyme thyroid peroxidase (T.P.O)

Question 6

formation of thyroinine, T3, T4:

Answer 6

The amino acid Tyrosine: also comes from the blood. It is necessary for the synthesis of a protein called Thyroglobulin. The condensation of two tyrosines forms Thyronine (formed of 2 aromatic rings connected by an ether bridge).
After binding active iodine, thyronine gives thyroid hormones T3 and T4 which differ from each other by the number of the iodine atoms they carry

Question 7

thyroglobulin role:

Answer 7

Thyroglobulin is a large glycoprotein produced and stored in the thyroid follicular cells of the thyroid gland. It serves as the precursor or scaffold for the synthesis of thyroid hormones
- Thyroglobulin contains many tyrosine residues, which are the sites for iodine attachment
- The iodinated tyrosine residues are then coupled to form thyroxine (T4) and triiodothyronine (T3), which remain bound to the thyroglobulin molecule.
- hyroglobulin acts as a storage form and source of thyroid hormones and is critical for the production of both T3 and T4.

Question 8

thyronine =

Answer 8

basic form of T3-T4

Question 9

synthesis of thyroglobulin:

Answer 9

synthesized in the ribosomes and then transported to the golgi and the endoplasmic reticulum. it contains the amino acids tyrosine and the thyronine
The thyroglobulin is internalized in vesicles, which move towards the apical pole, where they discharge the thyroglobulin at the level of the colloid, after their fusion with the plasma membrane

Question 10

Tyrosine amino acid iodination of thyroglobulin:

Answer 10

• Active iodine binds to tyrosine amino acids
  of thyroglobulin :
• under the action of thyroid peroxidase (TPO)
• at the apical pole of the thyreocyte
• The bond of one or two iodine atoms on tyrosine
  will give the precursors of thyroid hormones:
  o Monoiodotyrosine (MIT)
  o Diodotyrosine (DIT)

Question 11

coupling of MIT and DIT:

Answer 11

TPO at the colloid level will bring together the two precursors:
o 2 x DIT gives T4 : Tetra Iodo-thyronine or Thyroxine (composed of 4 iodine atoms)
o DIT+MIT gives T3 : Triodo-thyronine (composed of 3 iodine atoms)
At this stage, thyroid hormones are still bound to thyroglobulin
During the formation of thyroid
hormones, the fixing of iodine atoms
can be done differently, concerning
T3, as you can see in the diagram opposite
o Reverse T3 is a form of T3, synthesized and secreted by the thyroid but which remains inactive

Question 12

Release of thyroid hormones:

Answer 12

In the event of stimulation of the
thyreocyte for the synthesis of
thyroid hormones, the
thyroglobulin enters, by
endocytosis, in the cytoplasm, then
in the lysosomes
Lysosomal enzymes separate thyroid hormones from thyroglobulin
Only T3, T4 and reverse T3 are secreted into the blood.
Iodine from MIT and DIT will be released by the cytoplasmic enzyme Deiodinase
to be recycled in the synthesis of new thyroid hormones

Question 13

T3 and T4 secretion:

Answer 13

a) T4 (tetraiodothyronine or thyroxine) :
o Main hormone secreted by the thyreocyte in the blood (90%)
o Part of T4 is transformed into T3, before secretion, by thyrocyte deiodinase
b) T3 (triiodothyronine)

Question 14

free form of thyroid hormone:

Answer 14

Only a very small fraction of the
hormone, circulates in free form (not bound to transport proteins)
It is this free hormone that is active
In biology, the determination of the concentrations of free thyroid hormones
(FT4 and FT3) is of great diagnostic importance.

Question 15

transport of thyroid hormones in the blood:

Answer 15

Thyroid hormones bind, immediately after their secretion, to plasma proteins:
* TBG (thyroxine binding globulins): the most dominant, Produced by the liver, binds T4 with very high affinity
* TBPA (Thyroxin-binding-prealbumin):
Binds mainly to T3, it has less affinity for T4
* Albumin : non-specific transport protein
for various hormones and other molecules, Thyroid-related hormones are inactive

Question 16

Binding of thyroid hormones to their specific receptors:

Answer 16

• Each receptor can bind only one hormone, which is specific to it
  (This is a generality that concerns all hormones)
• • T3 is triangular,
    its receptor too
• T4 is circular,
  its receptor also

Question 17

conversion of T3 from T4:

Answer 17

Most T3 is produced in peripheral tissues. Indeed, T4 (that is abundant) is converted to
active T3, in peripheral cells, by the enzyme deiodinase
(T3 is 10 times more active than T4 and binds much more easily to peripheral receptors, for which it has a very high affinity)