Research Update: Are we overdiagnosing hypothyroidism in sighthounds?
Despite these two well-documented conditions—euthyroid sick syndrome and lower serum T4 concentrations in greyhounds—hypothyroidism is oftentimes diagnosed inappropriately in dogs with concurrent diseases or in greyhounds in general despite a lack of consistent clinical signs. The authors of this study sought to characterize the thyroid axis test results that are being used to diagnose hypothyroidism in sighthounds (the ancestral breed group to which greyhounds belong) and determine whether the lower T4 concentrations noted in greyhounds are also found in healthy Salukis, another sighthound breed.
The retrospective arm of this study reviewed submissions to the endocrinology department of the Diagnostic Center for Population and Animal Health at Michigan State University for serum thyroid hormone measurement over a period of 4.5 years. Cases were included if the dog was a sighthound and test results were subsequently interpreted by the requesting veterinarian as being consistent with hypothyroidism, thereby justifying levothyroxine supplementation. After excluding dogs that were receiving exogenous thyroid hormone supplementation at the time of testing, 398 dogs were included.Of these dogs, hypothyroidism was diagnosed in 30 (7.5%) despite all thyroid hormone concentrations falling within the reference range, and 286 dogs (71.9%) had either T4 or T3 concentrations below the general dog population reference range, but the thyroid-stimulating hormone (TSH) concentration was either within the reference range (and, therefore, most consistent with euthyroid sick syndrome) or had not been measured. Only 65 dogs (16.3%) had a decreased serum T4 or T3 concentration and either had an increased TSH concentration or thyroglobulin autoantibodies, either of which are more specific for a diagnosis of hypothyroidism when present in conjunction with a decreased serum T4 or T3 concentration.
The prospective cross-sectional portion of this study evaluated five thyroid-associated hormones in 283 healthy Salukis ranging in age from 12 to 167 months and compared these results to the laboratory reference ranges for the general dog population. All Salukis included in this study were determined to be healthy based on owner interviews and the results of physical examinations conducted at the time of blood collection. Dogs were excluded if they were receiving medications that had previously been shown to alter thyroid hormone concentrations, such as corticosteroids and anticonvulsants.
Of these Salukis, 154 of 282 (54.6%) and 120 of 281 (42.7%) had serum T4 or serum T3 concentrations, respectively, below the laboratory reference range. Despite the frequent finding of decreased serum T4 concentrations, only 25 Salukis (8.9%) had concurrent increases in serum TSH concentration, four of which also had thyroglobulin autoantibodies above the upper limit of the laboratory's reference range. Male Salukis had significantly lower T4 and free T4 (fT4) concentrations than females did, and T4, fT4, and TSH concentrations were correlated with both age and body weight.