Page 109 - Clinical Small Animal Internal Medicine

10 Feline Hyperthyroidism 77

no significant differences in radiographically defined Treatment
VetBooks.ir cardiac size between the two groups, a larger number of Once hyperthyroidism has been diagnosed, all manage­
cats in the 1982 group had evidence of congestive heart
failure. These findings suggest that feline hyperthyroid­
ism is being diagnosed earlier and with less severe clini­ ment options (thyroidectomy, radioactive iodine,
antithyroid drugs, nutritional management) should
cal signs than when studied a decade ago. be discussed with pet owners. All options can be ≥90%
effective for controlling hyperthyroidism when used
appropriately. The selected management option will
Diagnosis differ for each cat based on several considerations
(Table 10.1). Radioactive iodine therapy is considered
Diagnosis most often is based on the presence of one the gold standard for treatment of hyperthyroidism but
or more typical clinical signs and increased serum total most pet owners currently opt for medical manage­
thyroxine (T4) concentration. However, up to 10% of ment. Until recently, this included oral or transdermal
all hyperthyroid cats and 40% of those with mild dis­ antithyroid drugs. Now nutritional management using
ease have serum T4 values within reference range. The a limited‐iodine food is another option for cats with
diagnosis of hyperthyroidism should not be excluded hyperthyroidism.
on the basis of a single normal serum T4 value, espe­
cially in a cat with typical clinical signs, a palpable thy­ Radioactive Iodine
roid nodule and serum T4 in the upper half of the
normal range. In these cases, serum free T4 (fT4), Radioiodine treatment is often considered the best
measured by equilibrium dialysis, may provide an option for many hyperthyroid cats because:
alternative means of diagnosing hyperthyroidism in it has the potential to eliminate a benign thyroid tumor
cats with normal serum total T4 values. Studies docu­ ● or abnormal thyroid tissue with a single treatment
ment that up to 20% of sick euthyroid cats can have it treats extrathyroidal thyroid tissue, which may occur
increased fT4 concentration. Therefore, it is most ● in 10–20% of hyperthyroid cats
appropriate and reliable to interpret the two values no general anesthesia is required
together. Mid‐to‐high reference range total T4 and ● reported side‐effects are minimal.
increased fT4 concentration are consistent with hyper­ ●
thyroidism. In contrast, low total T4 4 and increased fT4 Cats should be stable prior to radioiodine therapy; those
values are usually associated with nonthyroidal illness. with clinically significant cardiovascular, renal, gastroin­
In cases where a strong index of suspicion exists for testinal, or endocrine (e.g., diabetes mellitus) disease
hyperthyroidism and test results are equivocal, thyroid may not be very good candidates, especially because of
imaging can be of benefit. the time necessary for boarding after treatment.

Table 10.1 Advantages/disadvantages of options for managing cats with hyperthyroidism

Option Advantages Disadvantages

Thyroidectomy Cures current tumor High initial costs
Requires anesthesia
Hospitalization required
Risk of postoperative hypocalcaemia
Irreversible
Radioactive iodine Cures current tumor High initial costs
Single treatment Limited availability
Effective for ectopic tissue Hospitalization required
Side‐effects uncommon Irreversible
Antithyroid drugs Routinely available Not curative (controls T4 and signs)
Reversible Daily administration needed
Costs spread over time Drug side‐effects
Limited‐iodine food Routinely available Not curative (controls T4 and signs)
Reversible Cat can only eat a single food
Costs spread over time