Objective—To determine the relationship between
surface emission rate of gamma radiation and urine
concentration of I131 (urine radioactivity) during the
period 7 to 21 days after oral or SC administration of
I131 to hyperthyroid cats.
Animals—47 hyperthyroid cats administered I131 PO
and 24 hyperthyroid cats administered I131 SC.
Procedure—A dose of I131 (1.78 to 2.04 X 102 MBq [4.8
to 5.5 mCi]) was administered orally. Surface emission
at the skin adjacent to the thyroid gland on days
7, 10, 14, 18, and 21 and number of counts/30 s in a
urine sample (1 mL, obtained via cystocentesis) on
days 7, 14, and 21 after oral administration were measured.
Effective half-life (T1/2E) was derived for each
point. Surface emission thresholds for maximum
urine radioactivity values were established. A dose of
I131 (1.48 X 102 MBq [4.0 mCi]) was administered SC.
Urine radioactivity and surface emission rates for SC
administration were compared with values for oral
Results—The T1/2E for surface emissions and urine
radioactivity progressively increased toward values
for physical T1/2 over time. The T1/2E for surface emissions
was 2.19 to 4.70 days, and T1/2E for urine
radioactivity was 2.16 to 3.67 days. Surface emission
rates had a clinically useful threshold relationship to
maximum urine concentrations of I131.
Conclusions and Clinical Relevance—Surface emission
rates for cats administered I131 appeared useful in
determining upper limits (threshold) of urine radioactivity
and are a valid method to assess the time at
which cats can be discharged after I131 administration.
(Am J Vet Res 2003;64:1242–1247)