Fortunately this doesn't apply to my cat because she is a borderline case and that is the problem. They are really not 100% sure she has it. All blood tests came back the same -- borderline.In lay terms, Scintigraphy is a diagnostic procedure done prior to the radioactive iodine injection. Since it is done before giving the injection it does not require the cat to stay longer. It helps the radiologist determine the size of the tumor on the thyroid so that they know the exact amount of radiation required to treat the Hyperthyroidism. Without the use of Scintigraphy they're merely guessing or giving all cats the same amount of radiation. I honestly don't know if it's new. But the radiologist also explained to me that if there are multiple tumors or extremely large tumors they would opt for removing the thyroid as opposed to doing the I-131.
Here is an article with a thorough explanation of Scintigraphy: Thyroid scintigraphy in hyperthyroidism - PubMed