Objective—To determine relationships between
commonly measured pretreatment variables and
duration of isolation for unrestricted dismissal after
oral administration of iodine 131 (131I) for treatment of
hyperthyroidism in cats.
Animals—149 hyperthyroid cats treated with 131I.
Procedure—A dose of 131I (2.9 to 6.04 mCi [1.07 to
2.23 × 108 Bq]) was administered orally to all cats after
hyperthyroidism was confirmed by evaluation of
serum total thyroxine (T4) concentrations. Forward
stepwise regression analysis was used to determine
whether pretreatment total T4 concentration, serum
creatinine concentration, body weight, age, 131I dose,
or concurrent administration of cardiac medication
(specifically excluding thyroid suppression drugs)
could be used as pretreatment predictors of duration
of isolation in a clinical setting. Gamma radiation
emission rate at dismissal was < 2.0 mR/h at skin surface
over the thyroid region.
Results—Mean ± SD duration of isolation was 16.67
± 4.42 days (95% confidence interval, 9.2 to 24.1
days). The regression equation for duration of isolation
calculated on the basis of dose of 131I (duration of isolation
[days] = 3.2 + [2.66 × mCi – 131I dose]) yielded a
regression line with a 95% confidence interval of
± 3.3 days; only 15% of the variation was explained.
Conclusions and Clinical Relevance—A pretreatment
estimate for duration of isolation could be determined
only from an equation based on the orally
administered dose of 131I. These findings suggest that
administration of the lowest efficacious dose possible
is the dominant factor in reduction of duration of isolation
for cats treated with 131I. (Am J Vet Res
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)