Failure of Radioiodine Therapy in Cats with Hyperthyroidism

Hyperthyroidism is common in cats, and various treatment options are available, including medication, diet, surgery, and radioactive iodine (RAI; ie, I¹³¹) therapy. RAI therapy is considered the treatment of choice because it is minimally invasive and typically achieves a permanent euthyroid state. RAI therapy has resolved hyperthyroidism in ≈95% of cats after a single treatment.¹

This study evaluated the outcomes of cats with persistent hyperthyroidism 2 to 4 weeks after receiving RAI therapy and assessed for predictors of ultimate treatment failure. Out of 959 treated cats, 121 continued to have hyperthyroidism (total thyroxine [tT₄] concentration, >3.1 µg/dL) 2 to 4 weeks after treatment; initial failure rate was 12.6%. Of these 121 cats, 87 had sufficient follow-up information, with 35 subsequently having tT₄ <3.1 µg/dL without further treatment. Most of the 35 cats achieved tT₄ concentration <3.1 µg/dL within 6 months (median, 8 months; range, 3-13 months).

Fifty-two cats continued to have hyperthyroidism after RAI therapy, and treatment was considered a failure; this failure rate (ie, 5.6%) is similar to previously reported rates.^2-5 Twelve of the 52 cats subsequently underwent thyroidectomy, of which only 4 showed resolution of hyperthyroidism. Of these 12 cats, 5 had thyroid carcinoma, 5 had adenoma, and 2 did not have histopathology.

Repeat RAI therapy was performed in 14 of the 52 cats in which treatment had failed, including one cat with failed thyroidectomy. Ten of the 14 cats had tT₄ concentration <3.1 µg/dL after repeat RAI therapy, and 2 cats later became euthyroid without further treatment, suggesting repeat RAI therapy can be successful in cats after initial treatment failure.

Higher tT₄ concentration at time of discharge and a higher weight-normalized RAI dose (ie, RAI dose divided by body weight) were independently predictive of treatment failure. tT₄ concentration >11.6 µg/dL had a 100% specificity for predicting failure.

Key pearls to put into practice:

RAI therapy successfully treats feline hyperthyroidism in ≈95% of cases and is generally considered the treatment of choice.

Approximately 40% of cats that remain hyperthyroid after an initial dose of RAI may have tT₄ concentration <3.1 µg/dL in a median of 8 months and not require further therapy. Cats may need to be monitored for ≥6 months after RAI therapy before repeat therapy is considered.

Treatment failure is likely in cats with posttreatment tT₄ concentration >11.6 µg/dL, and repeat RAI therapy should be considered earlier than 6 months.