[Thyroid hormone profiles in euthyroid, hypothyroid dogs, and dogs with defined non-thyroidal illness syndrome]

Thyroxine (T4) is routinely measured in dogs by some laboratories, and the result may be misleading as total thyroxine values (TT4) below the reference range are frequently observed. This may be due to either hypothyroidism or a non-thyroidal illness (Non-Thyroidal Illness Syndrome, NTIS). The aim of this study was to systematically investigate and compare TT4, free T4 (fT4), thyroid-stimulating hormone (TSH), and total triiodothyronine (TT3) concentrations, as well as thyroglobulin autoantibodies (TgAA) in dogs affected by NTIS as well as healthy and known hypothyroid dogs.

In this retrospective cross-sectional study, data from a medical database and 272 leftover serum samples from dogs presented at the Small Animal Clinic of the University of Veterinary Medicine Hannover between May 2021 and March 2023 were analyzed. In addition to samples from healthy dogs (n=40), a total of 463 diagnoses from 232 dogs were recorded and grouped. Multiple linear regression analyses were performed in the NTIS subsample, considering age as a covariate and various disease categories as predictors. Additionally, covariance analyses examined the effects of inflammation, multimorbidity, and hospitalization. Agreement between TT4 and fT4 was assessed by Spearman correlation and Bland-Altman analysis. The influence of phenobarbital therapy was investigated in the subgroup of dogs with epilepsy.

A total of 37 cases of hypothyroidism were diagnosed. Within the NTIS group, the most common diagnostic categories were nervous system diseases (n=125), gastrointestinal diseases (n=63), diseases of the urogenital tract (n=33), heart diseases (n=32), and oncologic diseases (n=31). Within 3 months, 6.6% of the dogs in all NTIS groups died. Lower TT4 and TT3 values were significantly associated with gastrointestinal diseases, hospitalization, more than 3 comorbidities, and inflammatory diseases, whereas fT4 was not. TT4 and fT4 levels were significantly reduced in dogs with NTIS and hypothyroidism compared to healthy dogs. Healthy dogs showed significantly lower log(TSH) values than dogs with hypothyroidism and significantly higher log(TSH) values than dogs with NTIS. Additionally, the log(TSH) values in dogs with hypothyroidism were significantly higher than those in NTIS dogs. For TT3 concentrations, the group comparison did not reveal any significant differences.

The study shows that dogs with NTIS have significantly lower TT4 and fT4 levels compared to healthy dogs. TSH concentrations in dogs with NTIS are significantly lower than in hypothyroid and healthy dogs. In addition, different diseases have varying degrees of influence on thyroid parameters.

The determination of TT4 and fT4 alone is not sufficient for the diagnosis of hypothyroidism, as NTIS may cause similar changes. The exclusion of other diseases is essential for the interpretation of thyroid hormone concentrations.