Long-term thyroid outcomes and survival in hyperthyroid cats treated with radioiodine: association with overt and subclinical hypothyroidism and levothyroxine treatment.

BACKGROUND

Long-term thyroid status and survival after radioiodine treatment of hyperthyroid cats are incompletely characterized, particularly for cats developing iatrogenic hypothyroidism.

HYPOTHESIS/

OBJECTIVES

To determine long-term thyroid outcomes and survival after radioiodine treatment, and to assess associations among thyroid status, hypothyroid remission, levothyroxine treatment, and survival.

ANIMALS: One thousand three hundred forty-four hyperthyroid cats treated with radioiodine.

METHODS

Prospective longitudinal cohort study. Thyroid and renal status were classified 6-12 months after radioiodine treatment and reassessed at 6- to 12-month intervals until death, euthanasia, relapse, loss to follow-up, or study completion. Kaplan-Meier analysis and Cox proportional hazards regression evaluated survival.

RESULTS

At 6-12 months, 1047/1344 cats (77.9%) were euthyroid, 240/1344 (17.9%) were subclinically hypothyroid, and 57/1344 (4.2%) were overtly hypothyroid. Among initially euthyroid cats, 934/1047 (89.2%) remained euthyroid, 43 (4.1%) developed late-onset hypothyroidism, and 70 (6.7%) developed recurrent hyperthyroidism. Among initially subclinically hypothyroid cats, 102/240 (42.5%) underwent remission to become euthyroid. Median survival was 49.7 months in euthyroid cats, 44.6 months in subclinically hypothyroid cats, and 36.1 months in overtly hypothyroid cats. In multivariable analysis, initial posttreatment thyroid status was not independently associated with survival, whereas older age, male sex, and higher creatinine concentration were. In final-outcome models, untreated subclinical and overt hypothyroidism were independently associated with an increased hazard of death, whereas hypothyroidism in cats treated with levothyroxine was not.

CONCLUSIONS AND CLINICAL

IMPORTANCE

Thyroid status after radioiodine treatment is dynamic. Subclinical hypothyroidism frequently resolves, whereas untreated persistent hypothyroidism is associated with shorter survival. Long-term monitoring of thyroid and renal function is critical in managing radioiodine-treated cats.