Thyroid scintigraphy is considered the gold standard test to confirm hyperthyroidism. In this test, a short-acting radionuclide that accumulates in the thyroid tissue is administered. A special machine (gamma camera – see below) is used to visualize where radionuclide has accumulated. This procedure uses thyroid physiology to create an image, which is why it is so sensitive in detecting what is actually happening to the thyroid tissue. Unfortunately, because of the cost of the machine, special training, and licensing, most radioiodine treatment facilities elect not to offer thyroid imaging for their hyperthyroid patients. Nonetheless, the use of thyroid scintigraphy is very important in cases of mild hyperthyroidism to confirm the diagnosis. It is also an important tool in the individualized I-131 dose determination protocol, as well as in cases of thyroid carcinomas, which require a much higher I-131 dose (up to 10 times the dose of a benign hyperthyroidism case).
Below are some examples of thyroid scintigraphy studies. It is intuitive to imagine that the more hyperactive tissue is affected, the more I-131 is required to effectively treat the patient. Unfortunately, there are many factors that play a role in feline hyperthyroidism, which include the variability of T4 secretion (T4 secretion can vary significantly throughout the day), the intensity of secretion of thyroid tissue, size of thyroid tumor, thyroid tumor’s ability to retain I-131, among others. Thus, the difficulty in finding one size fits all dosing scheme. Therefore, several parameters are now used to determine the ideal dose for each patient. The final dose is the average of the calculated dose for each parameter. This has significantly improved the success rate, especially in lowering the prevalence of hypothyroidism post-I-131 treatment.
*SHIM-RAD (severe elevations to T4, huge tumor size, intrathoracic nodules, multifocal disease, and refractory to treatment with antithyroid drugs)
Thyroid scintigraphy is non-invasive, only requires a few minutes, and does not typically require any sedation.