Efficacy of Low‐dose (2 millicurie) versus Standard‐dose (4 millicurie) Radioiodine Treatment for Cats with Mild‐to‐Moderate Hyperthyroidism

Abstract

BackgroundRadioiodine (131I) is effective treatment for hyperthyroidism in cats, but optimal dose to restore euthyroidism without inducing hypothyroidism is unclear. Treatment‐induced hypothyroidism can lead to azotemia and reduced duration of survival.ObjectiveTo compare efficacy and short‐term outcomes of low‐dose 131I versus higher, standard‐dose 131I as treatment for hyperthyroidism.AnimalsA total of 189 client‐owned cats undergoing 131I treatment for mild‐to‐moderate hyperthyroidism (serum T4 ≥ 4.0 μg/dL and <13.0 μg/dL).MethodsProspective, nonrandomized, cohort study comparing treatment with either low‐dose (2 mCi, n = 150) or standard‐dose (4 mCi, n = 39) 131I. Serum T4, thyroid‐stimulating hormone (TSH), and creatinine concentrations were measured after 1, 3, and 6 months to determine persistent hyperthyroidism, overt hypothyroidism (low T4, high TSH), subclinical hypothyroidism (normal T4, high TSH), and azotemia.ResultsThere was no significant difference in prevalence of cats with persistent hyperthyroidism between standard‐ and low‐dose treatment groups at 3 (0% versus 5.3%; P = .34) and 6 (0% versus 3.3%; P = .51) months. Overt (18% versus 1%; P = .0005) or subclinical (46% versus 21%; P = .004) hypothyroidism was more common in cats at 6 months after standard‐dose 131I. No difference in incidence of azotemia existed between groups, but cats treated with standard‐dose 131I had higher creatinine concentrations (P < .05) and higher percent rises in creatinine (P < .0001).Conclusions and Clinical ImportanceLow‐dose 131I is safe and effective for cats with mild‐to‐moderate hyperthyroidism, as evidenced by a cure rate of >95% with reduced frequency of iatrogenic hypothyroidism and azotemia.

Evaluation of Serum Symmetric Dimethylarginine Concentration as a Marker for Masked Chronic Kidney Disease in Cats With Hyperthyroidism

“…Only 5 (1.9%) cats developed overt hypothyroidism (low T 4 with high TSH concentration), resulting in a much lower prevalence of overt hypothyroidism than that reported after administration of higher 131 I doses . The prevalence of subclinical and overt hypothyroidism in the cats of this study is similar to those that we have previously reported in other cats treated with low‐dose 131 I …”

“…Additionally, the proportion of cats that develop azotemia after treatment for hyperthyroidism depends, at least in part, on the degree to which the hyperthyroidism is controlled (ie, how low the serum T 4 falls) . Importantly, azotemia is more likely to develop in cats with overt hypothyroidism than in cats with subclinical hypothyroidism or in 131 I‐treated cats that become euthyroid . As in previous studies, these cats with overt hypothyroidism had a higher prevalence of azotemia (80%) than did the cats with milder, subclinical hypothyroidism (31%).…”

“…All other cats were defined as nonazotemic (serum creatinine concentration ≤2.1 mg/dL). We also classified the cats’ thyroid status as euthyroid (both serum T 4 and TSH concentrations within reference interval), overtly hypothyroid (low serum T 4 with high TSH concentration), or subclinically hypothyroid (low‐normal serum T 4 with high serum TSH concentrations), as previously defined (Fig ) …”

“…Serum urea nitrogen was measured by an enzymatic method, and serum creatinine was measured by a modified Jaffe method, both with an automatized biochemistry analyzer . Serum T 4 and TSH were determined by assays validated for use in cats, as previously described …”

Thyroid and renal function in cats following low‐dose radioiodine (111Mbq) therapy

“…It is important to note that a higher cut‐off for TSH to define overt hypothyroidism was used compared to our current report (0.3 versus 0.15 ng/mL) (Lucy et al . ). In the same previous study, the incidence of subclinical hypothyroidism was 46% in cats treated with standard dose RAI and 21% in cats treated with low‐dose RAI.…”