Radioisotope imaging

From Cat

Radioactive-iodine uptake imaging is an alternative, and highly sophisticated, method of diagnosing hyperthyroidism in cats.

Hyperthyroid cats usually exhibit an increased thyroidal uptake of radioactive iodine (123I or 131I) or technetium-99m as pertechnate (99mTcO4-)[1]. Percentage uptake or increased thyroid:salivary ratio may be calculated, and both are correlated strongly with circulating thyroid hormone concentration and provide an extremely sensitive means of diagnosing hyperthyroidism[2].

Apart from the expense and the difficulties of dealing with radioisotopes, sophisticated and computerized equipment is required. In addition, anaesthesia is usually required, and caution is advised in interpreting results from cats previously treated with antithyroid drugs because radioisotope uptake can be enhanced for several weeks after drug withdrawal[3]. Qualitative thyroid imaging is most useful in assessment of thyroid involvement before surgical thyroidectomy. Similarly, high-resolution ultrasonography may provide an alternative to thyroid imaging, but a;rger studies are required for this evaluation.

Although thyroid uptake measurements and imaging may be useful in some cats with suspected hyperthyroidism, the diagnosis almost always can be made more easily and in a manner that is less stressful for the cat by measuring serum total thyroxine (T4) and triiodothyronine (T3) levels. If serum concentrations of T4 and free T4 are within reference ranges, clinical hyperthyroidism is highly unlikely and thyroidal uptake measurements or thyroid imaging probably are not warranted in such cats.

References

  1. August, JR (2006) Consultations in feline internal medicine. Vol 5. Elsevier Saunders, USA. pp:196
  2. Daniel, GB et al (2002) Quantitative thyroid scintigraphy as a predictor of serum thyroxin concentration in normal and hyperthyroid cats. Vet Radiol Ultrasound 43:374-382
  3. Nieckarz, JA & Daniel, GB (2001) The effect of methimazole on thyroid uptake of pertechnate and radioiodine in normal cats. Vet Radiol Ultrasound 42:448-457