Association of the risk of development of hypothyroidism after iodine 131 treatment with the pretreatment pattern of sodium pertechnetate Tc 99m uptake in the thyroid gland in cats with hyperthyroidism: 165 cases (1990-2002).
OBJECTIVE
To assess whether the risk of development of hypothyroidism after treatment with iodine 131 (131I) was associated with the pattern of sodium pertechnetate Tc 99m activity in the thyroid gland detected via scintigraphy before treatment in cats with hyperthyroidism.


METHODS
Retrospective study.


METHODS
165 cats.


METHODS
Medical records of cats with hyperthyroidism that had been treated with 131I (from 1990 to 2002) and had undergone scintigraphy of the thyroid gland before treatment were reviewed; data regarding signalment, scintigraphic findings (classified as unilateral, bilateral-asymmetric, bilateral-symmetric, or multifocal patterns), serum total thyroxine (T4) concentrations before treatment and prior to hospital discharge, and 131I treatment were collected. A questionnaire was sent to each referring veterinarian to obtain additional data including whether the cats subsequently developed hypothyroidism (defined as serum total T4 concentration less than the lower reference limit > or = 3 months after treatment).


RESULTS
50 of 165 (30.3%) 131I-treated cats developed hypothyroidism. Hypothyroidism developed in 39 of 109 cats with bilateral, 10 of 50 cats with unilateral, and 1 of 6 cats with multifocal scintigraphic patterns of their thyroid glands. Cats with a bilateral scintigraphic pattern were approximately 2 times as likely to develop hypothyroidism after 131I treatment than were cats with a unilateral scintigraphic pattern (hazard ratio, 2.1; 95% confidence interval, 1.04 to 4.2).


CONCLUSIONS
Cats with hyperthyroidism that have a bilateral scintigraphic pattern in the thyroid gland before 131I treatment appear to have a significantly higher risk of subsequently developing hypothyroidism, compared with cats with a unilateral scintigraphic pattern.
Nykamp
Stephanie G
SG
Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853, USA.
Dykes
Nathen L
NL
Zarfoss
Mitzi K
MK
Scarlett
Janet M
JM
D000818:Animals; D002371:Cat Diseases / Q000139:chemically induced* / Q000532:radiotherapy; D002415:Cats; D005260:Female; D006980:Hyperthyroidism / Q000532:radiotherapy / Q000662:veterinary; D007037:Hypothyroidism / Q000139:chemically induced / Q000662:veterinary*; D007457:Iodine Radioisotopes / Q000009:adverse effects* / Q000627:therapeutic use; D008297:Male; D011877:Radionuclide Imaging; D012189:Retrospective Studies; D012307:Risk Factors; D013670:Sodium Pertechnetate Tc 99m; D013961:Thyroid Gland / Q000000981:diagnostic imaging*; D013963:Thyroid Hormones / Q000097:blood
D007457:Iodine Radioisotopes; D013963:Thyroid Hormones; D013670:Sodium Pertechnetate Tc 99m