Choosing the best tests to diagnose feline hyperthyroidism - Veterinary Medicine
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Choosing the best tests to diagnose feline hyperthyroidism
Not all hyperthyroid cats exhibit clinical signs, and concurrent illness may skew the laboratory assessment of thyroid function. Here's the latest research on what tests to perform to establish a diagnosis.


VETERINARY MEDICINE



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.
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.

Thyroid scanning


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.
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

SUMMARY


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.
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.

David S. Bruyette, DVM, DACVIM
VCA West Los Angeles Animal Hospital
1818 S. Sepulveda Blvd.
West Los Angeles, CA 90025

REFERENCES

1. Broussard, J.D. et al.: Changes in clinical and laboratory findings in cats with hyperthyroidism from 1983 to 1993. JAVMA 206 (3):302-305; 1995.

2. Norsworthy, G.D. et al.: Palpable thyroid and parathyroid nodules in asymptomatic cats. J. Feline Med. Surg. 4 (3):145-151; 2002.

3. Fox, P.R. et al.: Electrocardiographic and radiographic changes in cats with hyperthyroidism: Comparison of populations evaluated during 1992-1993 vs. 1979-1982. JAAHA 35 (1):27-31; 1999.

4. Mooney, C.T. et al.: Effect of illness not associated with the thyroid gland on serum total and free thyroxine concentrations in cats. JAVMA 208 (12):2004-2008; 1996.

5. Peterson, M.E. et al.: Measurement of serum concentrations of free thyroxine, total thyroxine, and total triiodothyronine in cats with hyperthyroidism and cats with nonthyroidal disease. JAVMA 218 (4):529-536; 2001.

6. Foster, D.J.; Thoday, K.L.: Tissue sources of serum alkaline phosphatase in 34 hyperthyroid cats: A qualitative and quantitative study. Res. Vet. Sci. 68 (1):89-94; 2000.

7. Archer, F.J.; Taylor, S.M.: Alkaline phosphatase bone isoenzyme and osteocalcin in the serum of hyperthyroid cats. Can. Vet. J. 12:735-739; 1996.

8. Barber, P.J.; Elliott, J.: Study of calcium homeostasis in feline hyperthyroidism. J. Small Anim. Pract. 37 (12):575-582; 1996.

9. Reusch, C.E.; Tomsa, K.: Serum fructosamine concentration in cats with overt hyperthyroidism. JAVMA 215 (9):1297-1300; 1999.

10. Graham, P.A. et al.: Serum fructosamine concentrations in hyperthyroid cats. Res. Vet. Sci. 67 (2):171-175; 1999.

11. Nieckarz, J.A.; Daniel, G.B.: The effect of methimazole on thyroid uptake of pertechnetate and radioiodine in normal cats. Vet. Radiol. Ultrasound 42 (5):448-457; 2001.


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