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


OBJECTIVE
To compare efficacy and short-term outcomes of low-dose131I versus higher, standard-dose131I as treatment for hyperthyroidism.


METHODS
A total of 189 client-owned cats undergoing131I treatment for mild-to-moderate hyperthyroidism (serum T4≥ 4.0 μg/dL and <13.0 μg/dL).


METHODS
Prospective, 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.


RESULTS
There 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-dose131I. No difference in incidence of azotemia existed between groups, but cats treated with standard-dose131I had higher creatinine concentrations (P < .05) and higher percent rises in creatinine (P < .0001).


CONCLUSIONS
Low-dose131I 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.
Departments of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY.
J M
JM
Lucy
Animal Endocrine Clinic, New York, NY.
M E
ME
Peterson
Departments of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY.
J F
JF
Randolph
Departments of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY.
P V
PV
Scrivani
Departments of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY.
M
M
Rishniw
Departments of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY.
D L
DL
Davignon
Departments of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY.
M S
MS
Thompson
Population Medicine and Diagnostic Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY.
J M
JM
Scarlett
D000818:Animals; D053099:Azotemia / Q000209:etiology / Q000662:veterinary; D002371:Cat Diseases / Q000532:radiotherapy*; D002415:Cats; D003404:Creatinine / Q000097:blood; D005260:Female; D006980:Hyperthyroidism / Q000532:radiotherapy / Q000662:veterinary*; D007037:Hypothyroidism / Q000209:etiology / Q000662:veterinary; D007457:Iodine Radioisotopes / Q000009:adverse effects / Q000627:therapeutic use*; D008297:Male; D011446:Prospective Studies; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D016896:Treatment Outcome
D007457:Iodine Radioisotopes; D013972:Thyrotropin; D003404:Creatinine; D013974:Thyroxine
131I; Feline; Radioactive iodine; Radiotherapy; Thyroid gland