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Background: Iatrogenic hypothyroidism can develop after radioiodine-I 131 (RAI) treatment of hyperthyroid cats and can be diagnosed using the thyroid stimulating hormone (TSH) stimulation test. Objectives: To assess the effect of noncritical illness on TSH stimulation test results in euthyroid and RAI-treated cats. To assess the correlation of low total-thyroxine (tT4), low free-thyroxine (fT4), and high TSH concentrations with TSH stimulation test results. Animals: Thirty-three euthyroid adult cats and 118 client-owned cats previously treated with RAI. Methods: Total-thyroxine, fT4, and TSH were measured, and a TSH stimulation test was performed in all cats. Euthyroid control cats were divided into apparently healthy and noncritical illness groups. RAI-treated cats were divided into RAI-hypothyroid (after-stimulation tT4 ≤ 1.5 μg/dL), RAI-euthyroid (after-stimulation tT4 ≥ 2.3 μg/dL OR after-stimulation tT4 1.5-2.3 μg/dL and before : after tT4 ratio > 1.5), and RAIequivocal (after stimulation tT4 1.5-2.3 μg/dL and tT4 ratio < 1.5) groups. Results: Noncritical illness did not significantly affect the tT4 following TSH stimulation in euthyroid (P = .38) or RAI-treated cats (P = .54). There were 21 cats in the RAI-equivocal group. Twenty-two (85%) RAI-hypothyroid cats (n = 26) and 10/71 (14%) of RAI-euthyroid cats had high TSH (≥0.3 ng/mL). Twenty-three (88%) RAIhypothyroid cats had low fT4 (<0.70 ng/dL). Of the 5 (7%) RAI-euthyroid cats with low fT4, only one also had high TSH. Only 5/26 (19%) RAI-hypothyroid cats had tT4 below the laboratory reference interval (<0.78 μg/dL).

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