Page 282 - Anatomy and Physiology of Farm Animals, 8th Edition

Endocrinology / 267

Iodinated
thyroglobulin storage
VetBooks.ir Thyroglobulin

Thyroglobulin
secretion endocytosis
lysosomes

Phagolysosome

Addition of
carbohydrate
moieties (Golgi)
Proteolysis
of thyroglobulin
Iodide
lysosomal
enzyme T 4
synthesis T 3
Thyroglobulin
synthesis (rER)

AA uptake

Figure 13-8. Biosynthesis of thyroglobulin (left) and its resorption, proteolysis, and secretion (right).
Events are depicted in two cells, but they do occur in the same cell. AA, amino acid; rER, rough endoplas­
mic reticulum. Source: Dellmann and Eurell, 1998. Reproduced with permission of John Wiley & Sons, Inc.
intracellular receptor affinity for T and the many other hormones, they do not show a
3
potential for T to be converted to T after significant diurnal rhythm. These stable
3
4
it enters target cells, many consider T to levels are primarily maintained by negative
3
be the more biologically important of the feedback of T and T at the level of the thy­
4
3
two hormones. rotrope cells of the adenohypophysis. The
Secretion of T and T from the thyroid is thyroid hormones have direct effects on the
3
4
a complex series of events that begins with thyrotropes to inhibit TSH synthesis and
the phagocytosis of thyroglobulin by follicu­ release. Thyrotrophin‐releasing hormone
lar cells. Endocytotic vesicles containing (TRH) from the hypothalamus is always
thyroglobulin then fuse with lysosomes that present and acts to promote TSH synthesis
contain enzymes necessary to degrade the and release from thyrotrope cells in the ade­
thyroglobulin and release free T and T nohypophysis, but its levels do not respond
3
4
from their storage form as part of thy­ to feedback regulation by T and T .
4
3
roglobulin molecules. The free T and T Almost all tissues of the body are target
4
3
are then secreted into the blood (Fig. 13‐8). tissues for thyroid hormones, for almost all
TSH from the adenohypophysis stimulates have receptors for them. In mature animals
follicular cells to synthesize thyroglobulin the most general effect of thyroid hormones
and to secrete T and T into the blood. is to increase overall oxygen consumption
4
3
Thus, the overall effect of TSH is to increase and heat production. The basal metabolic
blood levels of the thyroid hormones. rate is a measure of oxygen use in resting
Plasma levels of thyroid hormones are conditions, so thyroid hormones are said to
relatively stable in adult animals; unlike increase it. The exact intracellular mechanisms