Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 20 Thyroid Function
the thyroidal follicular cells. After synthesis, it moves to
the cell membrane where the iodinations occur and the
iodinated thyroglobulin, or colloid, is released into the
lumen of the follicle where it is stored. The amount stored
can be quite large as evidenced by the mass of protein contained
within a normal follicle.
4 . Release of Hormone
TSH stimulation of the release of hormones is the second
of its two principal sites of action. Within a few minutes
after giving TSH, small packets of colloid are taken up by
the follicular cell membrane, and vesicles are formed and
taken into the follicular cell by endocytosis. The vesicles
merge with lysosomes within the cell, and the lysosomal
proteases hydrolyze the colloid and release their MIT, DIT,
T 3 , and T 4 . The released MIT and DIT are enzymatically
degraded by microsomal tyrosine deiodinases, and their
iodine is recycled within the follicular cell. The T 4 and T 3
are released into the circulation by a simple diffusion process.
Of the total hormone released, about 90% is T 4 and
10% is T 3 . Within the gland, there is also some deiodination
of the T 4 of the inner phenyl group to form rT 3 , but
most of this deiodination occurs in the peripheral tissues.
This rT 3 is the inactive form of thyroid hormone and is on
a degradation pathway.
VI . TRANSPORT OF HORMONE:
PROTEIN BINDING
The thyroid hormones in the circulation are T 4 , T 3 , and rT 3 .
Immediately upon entering the circulation, these hormones
are bound to transport proteins, mainly to thyroxine-binding
globulin (TBG), and with lesser amounts to thyroxine
binding prealbumin (TBPA) and to albumin. There is
a wide spectrum of species variation in hormone binding
by serum proteins ( Table 20-1 ). TBPA is present in all the
species in contrast to earlier reports indicating that TBPA
was present only in humans, rhesus monkey, horse, cat,
rabbit, pigeon, and chicken. The early work was based on
electrophoretic migration, but later work based on chemical
properties identified TBPA or its analogues at different
migration sites ( Larsson et al. , 1985 ). The differences in
migration are likely due to differences in the amino acid
composition of these proteins among the various species.
Using chemical criteria, all species have TBPA.
TABLE 20-1 Thyroxine-Binding Proteins of Various Species, Their
Electrophoretic Migration, and Their Relative Thyroxine Binding a
Electrophoretic Migration Position
Species Alpha-Globulin Albumin Prealbumin
Human TBG (73) ALB (10) TBPA (17)
Rhesus monkey TBG (59) ALB (8) TBPA (33)
Cattle * TBG (60) TBPA (20) ALB (20)
Sheep TBG (86) TBPA (14)
Goat TBG (63) TBPA (37)
Water buffalo TBG (78) TBPA (22)
Horse TBG (61) ALB (17) TBPA (22)
Pig TBPA (0) TBG (93) ALB (7)
Dog TBG (60) TBPA (17) α -G (11) ALB (12)
Cat * TBPA (39) ALB (61)
Rabbit * TBPA (73) ALB (27)
Rat * ALB (80) TBPA (20)
Mouse * ALB (80) TBPA (20)
Guinea pig * ALB (81) TBPA (19)
Chicken TBPA (10) ALB (75) TBPA (15)
Pigeon * ALB (50) TBPA (50)
a
TBG, thyroxine-binding globulin; ALB, albumin; TBPA, thyroxine-binding prealbumin; α-G, α-globulin:
positioned as shown relative to the albumin position in each species. Numbers in parentheses specify the percentage of
thyroxine binding. Constructed from the data of Tanabe et al. (1969), Refetoff et al. (1970), and Larsson et al.
(1985). The * indicates approximations.