Page 133 - BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery, 2nd Edition

BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery

the common carotid artery; it gives a branch to the exter- increase in total and ionized blood calcium. The para-
nal parathyroid gland as it enters the cranial pole of the thyroid gland responds directly to circulating serum
VetBooks.ir artery on the dorsal surface of the thyroid. The caudal minor role in calcium homeostasis. Circulating phosphate
calcium levels, regulating the release of PTH.
thyroid gland and anastamoses with the caudal thyroid
Calcitonin is secreted by the thyroid gland and has a
thyroid artery arises from the brachiocephalic trunk and
travels up the neck along the trachea, but is not always
indirectly cause release of PTH, as high levels of phos-
present in the cat. The cranial and caudal thyroid veins levels have no direct effect on PTH release, but they may
drain into the internal jugular vein. Lymphatic drainage is to phate may depress serum calcium levels. There is also a
the retropharyngeal and deep cervical lymph nodes, but limited direct response to serum magnesium.
can also occur directly into the cervical lymphatic trunk or Some of the effects of PTH are mediated by vitamin D.
internal jugular vein. PTH controls the final metabolism of vitamin D in the
The thyroid gland produces and stores thyroglobulin, kidney to 1,25-dihydroxyvitamin D3, which promotes intes-
which is the precursor of active thyroxine. Thyroid- tinal absorption of calcium and bone calcium resorption.
stimulating hormone (TSH) is released from the pituitary Receptors for 1,25-dihydroxyvitamin D3 in the parathyroid
gland to stimulate trapping of iodide by the thyroid gland gland provide another negative feedback mechanism,
and subsequent formation of triiodothyronine (T3) and thy- inhibiting PTH synthesis.
roxine (T4) from iodination of thyroglobulin. T4 is the
predominant hormone secreted and >99% of this is
protein-bound. It is deiodinated peripherally to give rise to Thyroid disorders
a more biologically active form, T3. Release of T3 and T4
is stimulated by TSH and results in an increase in the over-
all metabolic rate and anabolic effects. Diagnostics
Owing to the superficial location of the thyroid gland,
Parathyroid glands enlarged glands are often palpable in the neck of the
conscious animal. Physical examination may reveal other
The parathyroid glands are smaller, closely related to the
signs suggestive of thyroid hormone abnormalities such as
thyroid glands (Figure 10.2), and produce parathyroid tachy- or bradycardia, restlessness or lethargy, and hair
hormone. There are four parathyroid glands: two are
coat abnormalities.
usually well demarcated and located on the outside of the Cervical masses can be assessed using ultrasono-
craniodorsal aspect of each thyroid gland; the other two
graphy. This also enables determination of uni- or bilateral
are embedded in the parenchyma of the thyroid gland, disease, and the extent of invasion in the case of neo-
usually in the caudal part. However, there are many
plastic disease. Computed tomographic (CT) scanning is
reported variations in the number and relationships of the preferable to determine the extent of local disease as well
thyroid and parathyroid glands. Ectopic parathyroid tissue
as allowing staging at the same time, assessing regional
has been reported in up to 50% of cats and, in various lymph nodes and pulmonary parenchyma for metastatic
studies, in between 6% and 100% of dogs.
disease. Surgical biopsy of large cervical masses may be
The vascular supply is directly linked to that of the difficult as thyroid masses in the dog are often extremely
thyroid gland. In cats, the cranial thyroid artery may release
vascular; therefore, fine-needle aspiration with ultrasound
a branch to the cranial parathyroid externally, or it may guidance may be safer. In the cat, confirmation of hyper-
pass through the thyroid parenchyma first, perforating the
thyroidism with clinical biochemistry is considered ade-
thyroid capsule to reach the external parathyroid gland. quate for diagnosis, and thyroid masses are rarely
Like ectopic thyroid tissue, the ectopic parathyroid tissue
biopsied or aspirated. Scintigraphy using Technetium
-99
may be found at any point along the ventral neck, thoracic can help to identify thyroid masses and may be particularly
inlet and mediastinum.
useful for identification of ectopic thyroid tissue. In the
PTH maintains blood calcium levels by stimulating dog, up to 90% of thyroid masses are carcinomas and
resorption of calcium by the renal tubules, mobilizing
frequently metastasize. In the cat, carcinomas account for
calcium from bone and augmenting intestinal absorption only 1–3% of thyroid masses; adenomatous hyperplasia
of calcium. High levels of circulating PTH cause an
accounts for the remaining 97–99%. Blood analysis may
yield results suggestive of hyper- or hypothyroid disease
(see below) but specific tests include T4 and TSH levels,
and TSH response.
peci c tests
Hyperthyroidism (thyrotoxicosis) is the most common
endocrine disorder in cats and is reported to affect up to 1
in 300 cats. Measurements of serum T3 and T4 vary with
species, time of day and levels of circulating binding
proteins; measurement of peripheral serum T3 can be
unhelpful because most T3 is intracellular. In some non-
thyroid diseases total serum T3 decreases, although this
does not cause clinical manifestations of hypothyroidism
and free T4 is often within normal limits (‘sick euthyroid
syndrome’). Scintigraphy may be useful to distinguish
The left thyroid gland of a dog has been elevated using
10.2 those cases with true hypothyroidism and those with a
forceps. The normal parathyroid gland is visible on the cranial
pole as a paler flat disc closely associated ith the thyroid gland. The falsely low T4.
caudal thyroid artery is clearly visible. In older cats that are hyperthyroid, total T4 may fluctu-
(© Davina Anderson) ate widely, and repeated tests may be required to confirm

124

Ch10 HNT.indd 124 31/08/2018 11:41