THYROIDAL RADIOISOTOPE UPTAKE IN EUTHYROID CATS - A COMPARISON BETWEEN I-131 AND (TCO4-)-TC-99M

Two thyroidal evaluation systems in euthyroid cats (n = 12) were compa red. A single, confirmed hyperthyroid cat was included for interest. F irstly thyroidal uptake of an intravenous bolus of approximately 111 M Bq (3 mCi) (TcO4-)-Tc-99M was estimated by using a scintillation camer a and calculating the ratio of thyroid to salivary activities at 20 mi n 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, nam ely 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 mu Ci) I-131 using a standard thyr oid uptake system, measured at 1, 2, 4, 6, 8, 10, 12, 24 and 48 h afte r 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:sali vary ratio of the parotid/mandibuIar complex and the radioiodine uptak e at 12 h.