Page 281 - Anatomy and Physiology of Farm Animals, 8th Edition

266 / Anatomy and Physiology of Farm Animals

VetBooks.ir HO O CH 2 CH COOH

NH 2
3,5,3 ,5 -Tetraiodothyronine
(Thyroxine)

HO O CH 2 CH COOH
NH 2
3,5,3 ,-Triiodothyronine
Figure 13-6. Thyroid hormones.

called colloid, which consists of a protein–
iodine complex, thyroglobulin. The hor­
mones T and T are stored in the colloid as
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iodinated tyrosine residues that are part of
thyroglobulin molecules (Fig. 13‐8). This
type of storage is unique among endocrine
glands.
Scattered among the follicular lining
cells and adjacent to them are a small sub­
set of thyroid cells, the C cells (parafollic-
ular cells). The C cells produce calcitonin,
a peptide hormone that lowers the blood
level of calcium by inhibiting the action of
osteoclasts. Calcitonin release is directly
Figure 13-7. Thyroid follicles of varying sizes regulated by the negative feedback of
and shapes are filled with colloid and lined with serum calcium concentration on C cells,
follicular cells. Source: Dellmann and Eurell, 1998. not by TSH. The physiologic importance
Reproduced with permission of John Wiley & of calcitonin in the overall regulation of
Sons, Inc. serum calcium concentration is minimal
compared with the role of parathyroid
pigs, the bulk of the gland lies primarily on hormone (discussed later).
the ventral aspect of the trachea rather than
being clearly divided into lateral lobes. Thyroxine and Triiodothyronine. Both
A connective tissue capsule covers the T and T are biologically active (bind to
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gland and gives rise to septa that divide thyroid hormone receptors), and most of
the substance of the thyroid and support the the circulating hormones are bound to
vasculature of the gland. Arterial blood plasma proteins. The T and T in the body
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supply to the thyroid and the associated fluids that are not protein bound are the
parathyroid glands (discussed later) arrives biologically active molecules, but they are
as branches of the common carotid artery. also subject to degradation. In many
Microscopically, the thyroid gland species, the plasma levels of T are much
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consists of follicles, spheres lined by a higher than those of T because the affinity
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simple epithelium of cells that ranges from of T for plasma proteins is greater than T .
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cuboidal to columnar (Fig. 13‐7). Thyroid The intracellular receptors for the thyroid
follicles are filled with the product of the hormones bind both T and T , but they
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follicular lining cells, a gel‐like substance have a higher affinity for T . Because of the
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