Does a Limited Iodine Diet Affect the Response to Radioactive Iodine Therapy in Hyperthyroid Cats?

Rowland A.

Conference Proceedings, (2016). American College of Veterinary Internal Medicine, Denver:


A retrospective study was performed to determine the outcome of radioactive iodine (131I) therapy in cats previously treated with a limited iodine diet (Hill’s Prescription Diet y/d®).

Medical records were searched for cats receiving 131I therapy after documented feeding of the prescription diet for at least 14 days. Cats were excluded if the prescription diet had been supplemented with other foods, or if methimazole had been administered concurrently with, or subsequent to, the prescription diet. Owners were contacted directly to provide missing dietary information.

Nine cats fulfilled the study criteria. The prescription diet was fed for a median of 91 days (range: 19–371). The interval between discontinuing the diet and administering 131I was 0 days in 3 cats, 4–5 days in 3 cats, and 16–24 days in 3 cats. Highest serum total thyroxine (T4) without therapy was used in calculating 131I dose; median highest T4 was 10.3 mcg/dl (range: 4.3–23.8 mcg/dl; reference range: 0.78–4.27 mcg/dl). The median dose of 131I administered was 3.64 mCi (range: 3.06–5.26 mCi). The cats were initially evaluated 22–57 days after 131I therapy and hyperthyroidism had resolved in all 9 cats, with a median post-therapy total T4 of 1.3 mcg/dl (range: 0.5–1.8 mcg/dl). Total T4 was below the reference range in 2 cats (0.5 and 0.55 mcg/dl) at the initial post-therapy evaluation, but was normal 22 and 34 days later (0.9 and 1.6 mcg/dl respectively).

In this small group of cats, pre-treatment with a limited iodine diet did not adversely affect the response to 131I therapy.