The prevalence of hypothyroidism in dogs ranges from 0.2% to 0.8%.1 Although thyroxine (T4) assays are widely available, the variability in serum T4 concentrations in healthy and sick dogs means that, unfortunately, no single, specific diagnostic test exists for hypothyroidism.
In addition to abnormal concentrations of one or more thyroid axis hormones, a hypothyroidism diagnosis also requires the
presence of consistent clinical signs that resolve with appropriate therapy. However, although low T4 concentrations are the most commonly noted hormone abnormality in dogs with hypothyroidism, any concurrent illness may result
in euthyroid sick syndrome, which refers to decreased circulating T4 and triiodothyronine (T3) concentrations despite a normally functioning thyroid gland.2 In addition, breed variability in normal serum T4 concentrations exists, with greyhounds known to have serum T4 concentrations less than that of the general dog population.3
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.