What radiosotopes are used for thyroid imaging?
The basis for this procedure is the unique physiology of the thyroid gland that results in the selective uptake of iodide by thyroid tissue (4-7). Although various isotopes of iodine are available for use in thyroid scintigraphy, their concurrent beta emission and associated local tissue damage (e.g., 131-I) or greater expense (e.g., 123-I), have limited their use in veterinary medicine.
The pertechnetate ion has a similar size, molecular shape, and charge compared to iodide, which results in its uptake by thyroid tissue. The radionuclide technetium-99m pertechnetate (99m-TcO4) is a pure gamma emitter with a low photon energy (140 KeV) that makes it ideal for diagnostic imaging (4-7). Because of these properties, as well as the fact that technetium is relatively inexpensive, it has become the radionuclide of choice for routine thyroid imaging in veterinary medicine.
How a thyroid scan is performed in cats
To perform thyroid imaging in cats, a small dose (3-4 millicuries of technetium is administered subcutaneously. Between 20 to 60 minutes later, the cats are laid on their abdomen (ventral view) or side (lateral view) while the gamma camera acquires the thyroid image. The scanning process itself usually takes less than a minute and generally does not require sedation (4-6).
|Fig 1: Thyroid scintigraphy |
in a normal cat
5 reasons why a thyroid scan is performed
There are five reasons why thyroid scintigraphy should be considered in any cat with suspected hyperthyroidism, especially before radioiodine treatment.
Reason 1— First, thyroid scintigraphy helps confirm the diagnosis of hyperthyroidism, which is very useful in cats in which a thyroid nodule cannot be palpated (4-9). Because thyroid scintigraphy directly visualizes functional thyroid tissue and the “uptake” of the radioisotope can be estimated by determining the thyroid:salivary ratio, thyroid imaging can diagnose hyperthyroidism before laboratory tests are consistently abnormal (Figure 2). Thyroid scintigraphy is considered the gold standard for diagnosing mild hyperthyroidism in cats.
Reason 2—Thyroid scintigraphy can also exclude the diagnosis of hyperthyroidism in euthyroid cats that have false-positive elevations in their serum T4 or free T4 values. Studies of cats with nonthyroidal illness (e.g., diabetes, renal, gastrointestinal, or liver disease) have shown that between 6% and 12% of these cats have falsely high serum free T4 values, despite the fact that they are not hyperthyroid (10,11).
In addition, routine screening of an apparently healthy senior cat occasionally reveals laboratory abnormalities that include slightly high total or free T4 concentrations, consistent with mild hyperthyroidism (12). As with sick cats with falsely high free T4 values, however, no thyroid nodule can be palpated in many of these cats and thyroid imaging may fail to confirm hyperthyroidism. Therefore, not every cat with a high total T4 or free T4 value is truly hyperthyroid, and treatment for hyperthyroidism would be contraindicated.
|Fig 3: Ectopic thyroid adenoma|
in the chest cavity of a hyperthyroid cat
In addition, thyroid images can locate large tumors that gravity has pulled into the thoracic cavity, which cannot be palpated on physical examination (4-9).
Reason 4— By providing a visual image of hyperfunctional thyroid tissue, thyroid scintigraphy allows for the determination of thyroid tumor mass or volume, which is useful in calculating each cat’s radioiodine dose (6,12,13). The goal of 131-I therapy is to restore euthyroidism with a single dose of radiation without producing hypothyroidism.
Recent research confirms that iatrogenic hypothyroidism contributes to the development of azotemia and shortened survival times in cats overtreated with radioiodine (14). To minimize the incidence of iatrogenic hypothyroidism, it is important to administer the lowest effective dose to each individual cat, rather than giving a fixed dose of radioiodine to all cats (12). Again, thyroid scintigraphy provides an excellent method for evaluating the size of the hyperfunctional thyroid tissue, which aids in determining the proper dose to treat the individual hyperthyroid cat.
Reason 5—Thyroid scintigraphy also provides valuable information in the diagnosis and evaluation of hyperthyroid cats with thyroid carcinoma (Figure 4). Our recent studies suggest that, although thyroid carcinoma is rare in cats with recently diagnosed hyperthyroidism, the prevalence of carcinoma progressively increases in cats treated long term with antithyroid medications. Of cats treated for longer than 4 years with medical treatment, over 20% had scintigraphic evidence of thyroid carcinoma (15).
|Fig 4: Thyroid images of 6 hyperthyroid cats with thyroid carcinoma. Notice the large tumor volumes, with extension of disease beyond the limits of the thyroid capsule into the chest cavity in all cases.|
|Fig 5: Thyroid images of a hyperthyroid cat with thyroid carcinoma before and after high-dose radioiodine treatment. |
Notice the complete ablation of all thyroid cancer tissue 2 months after treatment (right).
No, a thyroid scan is not absolutely mandatory — most treatment facilities in the USA do not do thyroid imaging prior to treatment.
Despite the valuable information obtained by performing thyroid scintigraphy, it is not required prior to radioiodine therapy, just as thoracic radiographs are not required prior to amputating a limb in a patient with a primary bone tumor or a preanesthetic laboratory screening is not required prior to dental prophylaxis or other elective procedures requiring anesthesia. Nevertheless, use of thyroid scintigraphy is considered good medicine because the findings can and do result in modification of the therapeutic dose, aid in the diagnosis of hyperthyroidism, and provide prognostic information.
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- Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD,Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2013;in press.
- Volckaert V, Vandermeulen E, Saunders JH, et al. Scintigraphic thyroid volume calculation in hyperthyroid cats. J Feline Med Surg 2012;14:889-894.
- Williams TL, Elliott J, Syme HM. Association of iatrogenic hypothyroidism with azotemia and reduced survival time in cats treated for hyperthyroidism. J Vet Intern Med 2010;24:1086-1092.
- Peterson ME, Broome MR: Hyperthyroid cats on long-term medical treatment show a progressive increase in the prevalence of large thyroid tumors, intrathoracic thyroid masses, and suspected thyroid carcinoma. J Vet Intern Med 26:1523,2012.