338
Chapter 11
OH
I
I
(DIT)
OH
I
(MIT)
Iodide is oxidized and
attached to rings of tyrosines
in thyroglobulin (TG)
I
I
OH
I
I
I
I
OH
I
(T
3
)
(T
4
)
The iodinated ring of one
MIT or DIT is added to a DIT
at another spot
Endocytosis of thyroglobulin
containing T
3
and T
4
molecules
Lysosomal
enzymes
release T
3
and T
4
from TG
Free amino
acids re-used
for TG synthesis
TG is synthesized in follicle
cell and secreted to colloid)
T
3
T
4
T
3
T
4
T
3
T
4
D
iffusion
3
2
4
5
6
7
1
Iodide is
cotransported
with Na
+
Lysosomes
TG
Follicle cell
Interstitial fluid
Capillary
Lumen of follicle (colloid)
RBC
I
–
Na
+
TG
Secretion
Figure 11–22
Steps involved in T
3
and T
4
formation. Steps are keyed to the text.
and, to a lesser extent, the hypothalamus (
Figure 11–23
). TSH
is also a trophic hormone, as stated earlier. TSH not only stimu-
lates T
3
and T
4
production, it also increases protein synthesis in
follicular cells, increases DNA replication and cell division, and
increases the amount of rough endoplasmic reticulum and other
cellular machinery required by follicular cells for protein synthe-
sis. Thus, if a thyroid cell is exposed to higher TSH levels than
normal, it will undergo
hypertrophy;
that is, it will increase in
size. An enlarged thyroid gland from any cause is called a
goiter
.
There are several ways in which goiters can occur, in addition
to increased exposure of the thyroid gland to TSH, as will be
described at the end of this section and in Chapter 19.
Actions of Thyroid Hormones
TH receptors are present in the nuclei of most of the cells of
the body, unlike receptors for many other hormones, whose
distribution is more limited. Thus, the actions of T
3
and
T
4
are widespread and affect many organs and tissues. The
receptors are located in the nucleus and can bind both T
3
and T
4
, but have a much higher affi nity for T
3
. Because most
of the T
4
that enters cells is deiodinated to T
3
, most receptor
binding sites are generally occupied by T
3
. As described in
Chapter 5, T
3
and T
4
act by inducing gene transcription and
protein synthesis.
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