Relationship between selected echocardiographic variables before and after radioiodine treatment in 91 hyperthyroid cats.

Ninety-one spontaneously hyperthyroid cats were studied by two-dimensional and M-mode echocardiography with in a week before and 2-3 months following oral radioiodine administration. A statistical search was made for a correlation between any of the linear echocardiographic variables (pre- or posttreatment) and the serum total thyroxine concentrations (pre- or posttreatment). No clinically useful relationships were found. Echocardiographic values were compared with a 95% confidence interval constructed from a previous multiinstitutional report on unsedated normal cats. Observed pretreatment abnormalities were primarily increases in interventricular septal and left ventricular wall thickness with reversion toward normal following treatment. The presence or absence of previous treatment with methimazole or concurrent treatment with cardiac-related drugs (beta-adrenergic blocker, calcium channel blocker, or angiotensin-converting enzyme inhibitor) had no statistically identifiable effect on the echocardiographic variables. About 37% of the cats had one or more echocardiographic variable(s) outside the calculated normal range before radioiodine treatment, but about 32% of the cats had one or more echocardiographic variable(s) outside that range after treatment (45% of these were normal before treatment). The conclusions are that pretreatment T-4 assessment was not useful in determining which cats may have potentially relevant echocardiographic abnormalities, that some echocardiographic abnormalities may emerge after treatment, and that less than 10% of the pre- or posttreatment abnormalities would be considered clinically relevant.

Relationship between orally administered dose, surface emission rate for gamma radiation, and urine radioactivity in radioiodine-treated hyperthyroid cats.

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 10(2) 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 10(2) MBq [4.0 mCi]) was administered SC. Urine radioactivity and surface emission rates for SC administration were compared with values for oral administration. 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.

Evaluation of relationships between pretreatment patient variables and duration of isolation for radioiodine-treated hyperthyroid cats.

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 x 10(8) 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 X 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.