608
UNIT 3
Regulation and Integration of the Body
16
the thyroglobulin is packed into transport vesicles. Tese
vesicles move to the apex of the follicular cell, where they
discharge their contents into the follicle lumen to become
part of the stored colloid.
2
Iodide is trapped.
±o produce the functional iodinated hor-
mones, the follicular cells must accumulate iodides (anions
of iodine, I
2
) from the blood. Iodide trapping depends on
active transport. (Te concentration of I
2
is over 30 times
higher inside the cell than in blood.) Once trapped inside
the follicular cell, iodide then moves into the follicle lumen
by facilitated diffusion.
3
Iodide is oxidized to iodine.
At the border of the follicular
cell and colloid, iodides are oxidized (by removal of elec-
trons) and converted to iodine (I
2
).
4
Iodine is attached to tyrosine.
Once formed, iodine is at-
tached to tyrosine amino acids that form part of the thy-
roglobulin colloid. Tis iodination reaction, mediated by
peroxidase enzymes, occurs at the junction of the follicular
Synthesis
Te thyroid gland is unique among the endocrine glands in its
ability to store its hormone extracellularly and in large quanti-
ties. A normal thyroid gland stores enough colloid to provide
normal levels of hormone for two to three months.
When ±SH from the anterior pituitary binds to receptors
on follicular cells, their
first
response is to secrete stored thy-
roid hormone. Teir
second
response is to begin synthesizing
more colloid to “restock” the follicle lumen. As a general rule,
±SH levels are lower during the day, peak just before sleep, and
remain high during the night. Consequently, thyroid hormone
release and synthesis follows a similar pattern.
Let’s examine how follicular cells synthesize thyroid hor-
mone
(Figure 16.10)
:
1
Thyroglobulin is synthesized and discharged into the folli-
cle lumen.
AFer being synthesized on the ribosomes of the
thyroid’s follicular cells, thyroglobulin is transported to the
Golgi apparatus, where sugar molecules are attached and
To peripheral tissues
T
3
T
3
T
3
T
4
T
4
Lysosome
Golgi
apparatus
Tyrosines (part of thyroglobulin molecule)
T
4
DIT
Iodine
MIT
Thyro-
globulin
colloid
Iodide (I
-
)
Rough
ER
Capillary
Colloid
Colloid in
lumen of
follicle
Thyroid follicular cells
Iodinated tyrosines are
linked together to form T
3
and T
4
.
5
Iodide
is oxidized
to iodine.
3
Thyroglobulin colloid is
endocytosed and combined
with a lysosome.
6
Lysosomal enzymes
cleave T
4
and T
3
from
thyroglobulin and hormones
diffuse into bloodstream.
7
Iodide
(I
-
) is trapped
(actively transported in).
2
Thyroglobulin is synthesized and
discharged into the follicle lumen.
1
Iodine is attached to tyrosine
in colloid, forming DIT and MIT.
4
Figure 16.10
Synthesis of thyroid hormone.
Only a few tyrosines of the thyroglobulins
in the colloid are illustrated. The unstructured yellow substance in the follicle lumen is colloid.
(MIT
5
monoiodotyrosine; DIT
5
diiodotyrosine.)
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