RESEARCH COMMUNICATIONS OF THE 28th ECVIM-CA CONGRESS

Thyroid autoantibodies are used as markers of autoimmune thyroiditis in humans and dogs. In humans, their presence is associated with a 2.5‐5.0% per year progression from subclinical thyroiditis to hypothyroidism. Several survey studies have shown an increased prevalence of positive thyroglobulin autoantibody (TgAA) in dogs with thyroid test results indicative of primary hypothyroidism versus euthyroid dogs. The implications of a positive TgAA result with otherwise normal concentrations of thyroid hormones and TSH (subclinical thyroiditis) on the subsequent progression to hypothyroidism in dogs are less clear. The objective of this study was to define the outcome of thyroid status in dogs with subclinical thyroiditis over a multi‐year interval. Purebred dogs with subclinical thyroiditis (>25% specific TgAA binding but with normal concentrations of T4, FT4 and TSH) were identified from the Orthopedic Foundation for Animals (OFA) Thyroid Registry database. Owners were invited to arrange submission of a follow‐up serum sample with their veterinarian along with inclusion of a medical history form including subsequent treatments. Serum samples were obtained from 114 dogs with subclinical thyroiditis that met inclusion criteria. The study population included 62 female and 52 male dogs from 30 breeds with English Setters, Boxers, Rhodesian Ridgebacks and Dalmatians most frequent. The average age at study completion was 6.3 years (SD = 2.2 years, range 2‐13). The mean follow‐up time was 3.9 years from initial testing (SD = 2.0 years, range 1‐9). At the time of retesting, 33% of the dogs had progressed to hypothyroidism, or were treated with thyroxine. Fifty‐one percent maintained positive or equivocal TgAA results while remaining euthyroid. In 16% of dogs, TgAA results had normalized with dogs remaining euthyroid. Initiation of thyroxine supplementation following a TgAA‐positive result was usually, but not always, based on suggestive clinical signs and decreased thyroid hormones at subsequent testing. These results suggest that the majority of dogs with elevated thyroglobulin autoantibodies either exhibit persistent autoimmune thyroiditis with continued risk of hypothyroidism or progress to hypothyroidism.