Does feeding an iodine-restricted diet have an effect on total T4 (TT4) concentrations and the signs of hyperthyroidism in cats with spontaneous hyperthyroidism? This was the question posed by the investigators in this study. Long-term treatment options for hyperthyroidism in cats usually include medical management with methimazole, treatment with radioactive iodine (I131), or surgery by thyroidectomy. Due to various complicating factors, these treatment options may not be available for some cat patients. The authors determined to evaluate further this novel therapeutic approach for the management of hyperthyroidism by the feeding and use of an iodine-restricted diet.
Forty-nine client owned cats (22 female, 27 male) with spontaneous hyperthyroidism were included in this retrospective study. Clinical signs most commonly noted were weight loss, increased thirst and urination, increased appetite, and vomiting, diarrhea or both. Nine of the 49 cats had been treated with methimazole but treatment had been discontinued due to various reasons. Before feeding the iodine-restricted diet, the median total T4 was 8.5 mg/dL. Clinical response was followed and assessed by a change in weight, heart rate along with serum TT4, blood urea nitrogen (BUN) and creatinine concentrations at different time periods (21-60 days, 61-180 days).
The results indicated that 20 of 48 cats had thyroid values within their reference range at 21-60 days of feeding the diet and 39 of 47 cats were within the reference range at 61-180 days. In cats followed longer than 180 days, 80% had serum TT4 concentrations within their respective reference ranges. Of the remaining cats outside the reference range, the majority was below the reference range and 3 were above.
The study shows that feeding an iodine-restricted diet to cats with spontaneous hyperthyroidism will achieve normal serum TT4 concentrations by 61-180 days after starting the diet. However, clinical signs such as weight loss or tachycardia (elevated heart rate) showed no improvement during the study period. The authors felt this suggests a lack of clinical control of the disease regardless of the serum TT4 concentration being within the normal reference range. Fortunately, even though the cats appeared to be in a physiologically hyperthyroid state, body weights did stabilize. In addition, cats taking a longer period of time to return to the reference range were more likely to have a higher TT4 concentration initially than cats whose TT4 concentration was within range by day 60. Therefore, it may take longer to return the TT4 to a normal range in cats with a higher initial TT4 concentration.
The authors highlighted the need for strict adherence to feeding the iodine-restricted diet with especially no additional treats or supplemental food given to a hyperthyroid cat patient on this therapeutic diet.
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