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

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Chapter | 20 Thyroid Function
a fairly wide band of glandular tissue, which forms the
connecting isthmus. In the horse, sheep, goat, cat, and
dog, the isthmus is a narrow remnant of tissue and may be
nonexistent. The size of the gland relative to body weight
is extremely small in all animals, approximating 0.20%
of body weight. The size of the gland varies and may be
enlarged when iodine deficiency, ingestion of goitrogenic
toxins, tumors, or hyperactivity is present. In primary
hypothyroidism, the gland may be reduced to fibrotic and
inactive remnants of thyroid tissue.
Accessory or extrathyroidal tissue is commonly observed
in the dog, particularly near the thoracic inlet though they
may be found anywhere along the esophagus. This accessory
tissue is fully functional physiologically, synthesizes
hormone, and can be located by its uptake of radionuclides.
This is a particularly important consideration for the dog
when thinking about surgical intervention for treatment of
hyperthyroidism resulting from a thyroid tumor.
The thyroid gland is a highly vascularized tissue with a
large blood flow. The functional unit of the thyroid gland
is the thyroid follicle, which can be envisioned as a spherical
structure composed of an outer monolayer of follicular
cells surrounding an inner core of colloid. Colloid is a
thyroglobulin-hormone complex that is the storage reservoir
of thyroid hormone in the thyroid gland. The colloid stored in
the lumen is a clear, viscus fluid. The individual follicular
cells vary from 5 to 10 μ m in height, and the entire follicle
may vary from 25 to 250 μ m in diameter. The size of the
follicles and the height of their follicular cells vary according
to the functional state of the gland. The cells may
vary from an inactive squamous cell to the highly active,
tall columnar cell. Interspersed between the follicles are
the thyroid “ C ” cells, the source of calcitonin, the hypocalcemic
hormone associated with calcium metabolism. A
third type of hormonal tissue, the parathyroid, is imbedded
within the thyroid or located in close proximity to it. The
parathyroids are the source of parathormone, the hypercalcemic
hormone. Removal of the parathyroids is virtually
unavoidable during surgical thyroidectomies so that postsurgical
hypocalcemias are important consequences to be
considered.
III . NUTRITIONAL REQUIREMENTS
The thyroid gland is unique among the endocrine glands
in that an integral part of its hormone, L-thyroxine (T 4 ), is
a trace mineral, iodine, which is available to the animal in
only limited amounts. Marine plants are known to be good
sources of iodine, but on land, iodine is limited and many
regions of the world are known to be iodine deficient. The
recommended daily requirement is 35μg/kg bw (276nmol/kg
bw) for the adult dog and 70 μg/kg bw (551.6nmol/kg bw)
for the growing puppy. For most animals, nutrient requirements
are given in mg/Kg dry diet or ppm and range from
0.1 to 1.0 mg, including cats and horses. Milk is a poor
source of hormonal iodide contributing only about 4% to
7% of the maintenance requirements for hormone ( Akasha
and Anderson, 1984 ).
The small requirements are compensated for by an efficient
intestinal absorption mechanism and conservation and
recycling of internal iodine. Little iodine is lost from the
body by the various excretory routes such as urine, saliva,
tears, milk, sweat, and feces. Also, whereas most endocrine
glands store little of their hormones, the thyroid manages to
store large quantities of hormone sufficient for 1 to 3 weeks
depending on the species. The thyroid gland contains about
20% of the total body iodine. Its iodine content and size
vary with iodine intake and the state of thyroid function,
but it usually contains 10 to 40mg (78.8 to 315.2μmol)
iodine/100 gm tissue or 4 to 16mg (31.5 to 126.1μmol)
iodine in a 20-kg dog.
IV . IODINE METABOLISM
Iodine can be absorbed in any of its soluble chemical
forms, but in the intestines it is usually absorbed in the
form of iodides (I ), iodates (IO 4 ), or as the hormonal
forms. Iodides may be absorbed from any moist body surface,
including the mucus membranes, and it is absorbed
easily through broken epithelia. Normally, the chief route
of entry into the general circulation is by absorption
through the mucosal cells of the small intestine. The I in
the circulation is trapped almost exclusively by the thyroid
gland, with small amounts being trapped by the salivary
gland and minimal amounts by the gastric mucosa, placenta,
and mammary gland. In the ruminant, 70% to 80%
of an oral dose is absorbed in the rumen and 10% in the
omasum.
The main route of excretion of I is by the kidneys
through which almost all the I that was not trapped by the
thyroid is lost in the urine. A small but significant amount
is lost in the saliva, and minimal amounts are lost in tears,
feces, sweat, and milk. A minute amount of free hormone,
that fraction not bound to serum proteins, is also lost in
the urine. These routes of excretion are especially important
considerations when patients are being treated with
radioiodine.
V . FUNCTIONS OF THE THYROID GLAND
The main functions of the thyroid gland are the trapping of
I and the synthesis, storage, and release of thyroid hormones
( Fig. 20-1 ), and these activities are under the control
of the thyrotropin or the thyroid-stimulating hormone
(TSH). Although TSH stimulates all steps in hormonogenesis,
the trapping of I and the release of hormone are the
two major sites of its action.