Thyroidal radioisotope uptake in euthyroid cats : a comparison between 131I and 99MTcO4

Abstract

Two thyroidal evaluation systems in euthyroid cats (n = 12) were compared. A single, confirmed hyperthyroid cat was included for interest. Firstly, thyroidal uptake of an intravenous bolus of approximately 111 MBq (3 mCi) 99MTcO4- was estimated by using a scintillation camera and calculating the ratio of thyroid to salivary activities at 20 min and 4 h. Thyroid to salivary activity ratios were 1:1 at 20 min and 2:1 at 4 h. Two discrete areas of salivary uptake were identified, namely a parotid/mandibular complex and a more rostral buccal/sublingual complex. These results were compared to radioiodine uptake of an oral dose of approximately 0.925 MBq (25 mCi) 131I using a standard thyroid uptake system, measured at 1, 2, 4, 6, 8, 10, 12, 24 and 48 h after administration. Mean radioiodine thyroidal uptake started at 33 % at 1 h, stabilised at 21 % between 4 and 24 h, and dropped to 18 % at 48 h. There was a significant correlation between the early thyroid:salivary ratio of the parotid/mandibular complex and the radioiodine uptake at 12 h.