In another study, fructosamine concentrations were evaluated in 30 nondiabetic hyperthyroid cats before and 30 days after
radioiodine treatment (discussed in the next article) and compared with normal control cats.10 Fructosamine concentrations were significantly lower in the hyperthyroid cats both before and after treatment with radioiodine.
However, treatment was associated with a statistically significant increase in fructosamine concentrations. This study, like
the study evaluating cats with diabetes mellitus, shows that fructosamine concentrations in hyperthyroid cats will be lower
than those in nonhyperthyroid cats (normal or diabetic) and that this effect is probably from the effects of hyperthyroidism
on increasing protein turnover.
Figure 1. Scintigrams of a 10-year-old domestic shorthaired cat with hyperthyroidism. Unilateral uptake of pertechnetate with
pooling of the isotope is evident in the heart along with the trapping of the isotope in the gastric mucosa. The area of interest
is highlighted on the right panel.
Scintigraphy has been used since 1985 to diagnose hyperthyroidism. The isotopes most commonly used for imaging are pertechnetate
(99mTcO4) and iodine-123 (123I). Hyperplastic thyroid glands have increased uptake of these radioactive isotopes, which is detectable by nuclear imaging.
This procedure detects whether one or both glands are enlarged (70% of cats have bilateral enlargement) and detects other
sites of thyroid gland activity (e.g. ectopic benign thyroid tissue, metastatic thyroid carcinoma) (Figures 1-3). Disadvantages include the lack of nuclear imaging centers available to practitioners, the need for anesthesia, and the
cost. Furthermore, recent data indicate that treating a cat with methimazole before nuclear imaging may increase the radioactive
material absorbed by the thyroid gland, creating false interpretations of increased thyroid activity.11
Figure 2. Scintigrams of an 11-year-old domestic shorthaired cat with hyperthyroidism. Bilaterally asymmetric uptake of pertechnetate
with mild pooling of the isotope is evident in the heart along with the trapping of the isotope in the gastric mucosa. The
area of interest is highlighted on the right panel.
Hyperthyroidism is a common endocrine disorder in senior cats. Clinical signs in cats with hyperthyroidism appear to be declining
in severity, likely because of earlier recognition of the disorder and the use of routine total T4 screening tests in senior cats. Nonthyroidal illness can interfere with the diagnosis of hyperthyroidism, especially in cats
with mild increases in total T4. Measuring free T4 by equilibrium dialysis and performing nuclear scintigraphy can enhance our ability to accurately diagnose this disorder
in cats, including cats with concurrent nonthyroidal illness.
Figure 3. Scintigrams of an 11-year-old domestic shorthaired cat with hyperthyroidism. Mild, bilaterally asymmetric uptake
of pertechnetate with mild pooling of the isotope is evident in the heart along with the trapping of the isotope in the gastric
mucosa. The area of interest is highlighted on the right panel.
David S. Bruyette, DVM, DACVIM
VCA West Los Angeles Animal Hospital
1818 S. Sepulveda Blvd.
West Los Angeles, CA 90025
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