Chun R, Garrett LD, Sherman A, et al.
Feline hyperthyroidism was first recognized in 1979 and is now the most frequently diagnosed endocrinopathy in cats. Many studies have evaluated dose and response to radioiodine therapy; however, no strong factors have been identified as predictors of treatment outcome, either from pretreatment or short-term follow-up data. The objectives of this retrospective study were to determine if specific factors are predictive of response to radioiodine therapy in hyperthyroid cats.
Information was abstracted from the medical records of 144 cats treated with radioiodine for the problem of hyperthyroidism between 4/92 and 12/98. The records were selected based on a diagnosis of hyperthyroidism by either elevated serum thyroxine levels or abnormal technetium scan, treatment with 4 miC radioiodine IV, and at least five follow-up serum thyroxine concentrations (at 1-week and 1-, 3-, 6-, and 12-months post therapy). Additional information obtained from the medical record included age and weight at diagnosis, physical examination findings, history and clinical signs, complete blood count, serum chemistry profile, urinalysis, extent of disease as assessed by nuclear scintigraphy, salivary to thyroid ratio and previous treatments. Statistical analysis was performed by SAS using least squares regression models to test for significance.
The median age of the treated cats was 12.95 years. Approximately 50% of the cats had been on methimazole, this therapy was discontinued in most cats at least 5 days before administration of radioiodine. Pretreatment thyroxine values ranged from 1 to 309 nmol/l (normal range 10 to 45.5 nmol/l). Pretreatment technetium scan ratios of thyroid to salivary gland ranged from 1.55:1 to 25.18:1 (normal range 1:1 to 1.29:1). Scan ratio results were significantly correlated with serum thyroxine values at pretreatment, and at one week, one month and three months post treatment (p=0.0001, 0.001, 0.004, 0.03, respectively). A greater scan ratio was associated with higher thyroxine levels at these time points, but not at 6 or 12 months post-therapy.
In a population of hyperthyroid cats that had wide variation in pre-treatment thyroxine values and thyroid to salivary gland ratios, no significant difference existed between thyroxine concentrations at 12 months post-therapy. There was a significant positive correlation between pre-treatment thyroxine values and thyroid to salivary gland ratios. The initial thyroid to salivary gland ratio is positively correlated with thyroxine levels in the pre-treatment and early post-treatment period, but not with long-term response to therapy.