Page 136 - BSAVA Manual of Canine and Feline Head, Neck and Thoracic Surgery, 2nd Edition

Chapter 10 · Surgery of the thyroid and parathyroid glands

antibody (ANA) (>50%). Most of these are noticed within in tissue (0.4–2 mm); therefore, adjacent thyroid, para-
the first 3 months of therapy and resolve fairly rapidly thyroid and cervical tissues are not affected during treat-
VetBooks.ir tions should be expected with long-term treatment, feedback, does not take up the 131 I and should gradually
ment. Normal thyroid tissue is suppressed by the negative
with dose reduction or drug withdrawal. Some complica-
recover over 3–6 months following treatment of the ade-
although in many cases the cat is elderly and long-term
131
therapy is not expected.
The main limitation is the ability of the owner to dose noma. The I is administered on licensed premises with a
single subcutaneous injection. The cat is then hospitalized
the cat effectively, particularly in the early stages of treat- until the I has decayed and been excreted to acceptable
131
ment when the disease may make the cat more difficult to levels, which takes about 10–14 days. The treatment dose
handle. However, a UK-based owner survey indicated that of 131 I is calculated for each individual cat and clinical
79% of owners were willing to medicate their cats twice hypothyroidism or recurrence of hyperthyroidism are rare
daily to achieve control of their clinical signs (Caney, 2013). complications. Cats that have thyroid carcinomas are
Medical management is also an extremely important treated with a higher dose and may need to be hospitalized
part of preparing a cat for surgical or radioiodine treat- for longer before they are considered safe to go home.
ment. Temporary resolution of the effects of the hyper-
thyroidism has a number of advantages. Surgical treatment: Adequate preoperative assessment
and preparation are important (see Medical treatment,
• Reversal of thyrotoxic hypertrophic cardiomyopathy, above). Thyroidectomy is a simple, if skilled, surgical pro-
resolution of dysrhythmias and desensitization of cedure, requiring no specialist instruments. Meticulous
cardiac muscle to decrease anaesthetic risk. Many cats technique is important, particularly if bilateral surgery is
will recover normal cardiac parameters in the 6 months required. There are four reported techniques.
following resolution of hyperthyroidism. In those cats
whose cardiac signs worsen or fail to resolve after • The extracapsular technique involves complete
treatment, there may be an underlying primary dissection of the enlarged thyroid gland, ligation of the
cardiomyopathy or thyroid hormone-induced structural cranial and caudal blood supply, and removal of the
damage. parathyroid glands. Hypoparathyroidism is a near
• Reduction of the GFR allows repeat urinalysis and certainty with the technique if bilateral thyroidectomy is
blood biochemistry to determine whether the cat has necessary, even if the procedures are done several
occult renal disease. Medical therapy is an important weeks apart.
tool as a trial treatment prior to using an irreversible • The intracapsular technique dissects all thyroid
therapy, such as surgery or radioiodine therapy, to parenchyma away, leaving the entire capsule and blood
monitor the response of the kidneys to a reduction in supply intact. This technique has a high rate of
GFR as the cat becomes euthyroid. The ‘development’ recurrence of hyperthyroidism due to residual thyroid
of biochemical evidence of renal disease should be tissue inside the capsule, but a low rate of
regarded as an ‘unmasking’ of the renal disease, and hypocalcaemia postoperatively.
• A modified extracapsular approach (see Operative
the long-term management of the cat should be
reassessed with regard to which of the two diseases is Technique 10.1) involves ligation of the caudal blood
supply and dissection of the thyroid gland, together
more severe. Medical management may be titrated to
keep both diseases under moderate control. with the capsule, up to the level of the external
parathyroid gland. At this point, bipolar cautery is used
• Cachexic or severely debilitated cats may put on
weight and improve condition prior to surgery. to minimize dissection underneath the external
parathyroid gland, and the capsule is released from the
parathyroid gland. This technique is difficult in some
PRACTICAL TIP
cats that have a closely attached parathyroid gland.
Preoperative medical therapy is an important part of • With the modified intracapsular approach (see
assessing the cat’s renal status. Some cats will go into Operative Technique 10.2), the thyroid gland is
overt renal failure if the hyperthyroidism is fully dissected away from inside the capsule and then the
resolved thyroid capsule is removed, leaving the capsule intact
at the level of the external parathyroid gland.
Prognosis: Postoperative complications after thyroid-
Radioactive iodine treatment: Treatment with 131 I is the ectomy using different surgical techniques have been com-
treatment of choice for functional thyroid tumours and can pared (Figure 10.5). Bilateral disease has been reported in
also be used for those cats with thyroid carcinoma; it is up to 70% of cats with hyperthyroidism, and bilateral
non-invasive and curative in 95% of cases. It will also treat surgery is commonly indicated. Depending on the surgical
unidentified ectopic adenomas. I emits beta and gamma technique (see Figure 10.5), there may be an increased
131
radiation and is selectively taken up by the most active risk of postoperative hypocalcaemia if bilateral thyroid-
thyroid tissue (i.e. in the adenoma). Beta particles account ectomy is carried out, and some surgeons prefer to stage
for 80% of the radiation and have a very short path length the surgery, with 6 weeks between procedures.

Technique Mild hypocalcaemia Severe hypocalcaemia Long-term Recurrence of hyperthyroidism
hypoparathyroidism (Welches et al., 1989)
Intracapsular None p to
odified intracapsular 33 13.3 None 0
odified e tracapsular 3 3. None not statistically significant

10.5 ostoperative complications associated ith different thyroidectomy techni ues in the cat.

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