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Feline Hyperthyroidism in Cats with Normal T4 Levels: The Importance of Scintigraphy

Gelatt-Nicholson K. and Neuwirth L.

Conference Proceedings, (1999). American College of Veterinary Radiology, Chicago, IL:

Purpose: To compare T4 levels and thyroid scintigraphy in the evaluation of feline hyperthyroidism. Emphasis was placed on the scintigraphic results of cats with normal T4 levels as well as evaluating a new method of scintigraphic analysis.
Methods: Retrospective study evaluating all cats undergoing thyroid scintigraphy at the University of Florida from 1991 through 1995. Recorded values included signalment, T4 levels, and thyroid scintigraphic results. Confirmation of hyperthyroidism was made via pathology (after thyroidectomy surgery) or response to treatment (medical or radioiodine therapy).
Results: Forty-nine cats were seen during this time period for thyroid scintigraphy, with three cats having repeat scans performed (52 total scans). The mean age was 13.5 years and 95% were neutered animals. Of these 49 cats, 10% were diagnosed as euthyroid. Of the 44 cats with positive scintigraphic results, approximately 30% had normal T4 levels. Cats with bilateral hyperthyroidism had mean T4 values approximately 2.5 times higher than those with unilateral hyperthyroidism. The mean T4 levels of cats with unilateral hyperthyroidism were within the normal range. Nine cats had their regions of interest re-drawn based on having focal hot spots within one of the thyroid glands. Of these nine, 56% were judged as being hyperthyroid in that gland and this diagnosis was later confirmed.
Conclusion: Thyroid scintigraphy is an invaluable diagnostic tool in suspected feline hyperthyroidism, especially in cases with normal T4 levels but classic clinical signs. The regions of interest drawn on scintigraphic images may need to be adjusted if focal hot spots of activity are noted for accurate diagnosis.