Hypoalbuminemia and altered thyroid parameters in dogs with chronic diarrhoea: Is protein loss driving TSH elevation?

Systemic effects of chronic gastrointestinal disease can complicate the interpretation of endocrine tests in dogs. In this study, thyroid function was evaluated in 70 dogs with chronic diarrhoea to investigate its association with serum albumin and digestive biomarkers. Dogs were classified according to albumin concentration: ≥ 28 g/L (Group A, normal albuminaemia, N = 24), 20-27 g/L (Group B, moderate hypoalbuminaemia, N = 25), and < 20 g/L (Group C, severe hypoalbuminaemia, N = 21). Total thyroxine (TT4), free thyroxine (FT4), canine thyroid-stimulating hormone (cTSH), TT4/FT4 ratio, albumin, total proteins, cobalamin, folate, and canine trypsin-like immunoreactivity (cTLI) were measured. Thyroid hormones, cobalamin, cTLI, and total protein were significantly lower in hypoalbuminaemic dogs (Groups B and C), while cTSH was higher in Group C. Albumin correlated positively with TT4 and FT4, and negatively with cTSH. Nine dogs (12.9 %) had elevated cTSH (≥0.5 µg/L), including six in Group C (28.5 %), and showed significantly lower albumin, globulin, cobalamin, TT4, FT4, and TT4/FT4 ratio compared to dogs with cTSH< 0.5 µg/L). On multivariate analysis, a decreased TT4/FT4 ratio and lower globulin concentration were independently associated with elevated cTSH. These findings suggest that alterations in thyroid parameters are common in dogs with chronic diarrhoea and hypoalbuminaemia, particularly in severe cases. Low TT4, FT4, and TT4/FT4 ratios, together with elevated cTSH, may reflect thyroid hormone binding protein loss and altered binding dynamics rather than primary hypothyroidism; however, complementary tests, such as antithyroglobulin antibody assessment and/or TSH stimulation tests, were not performed, so subclinical primary hypothyroidism cannot be definitively ruled out. Such results highlight the risk of misdiagnosing hypothyroidism and underscore the need for cautious interpretation of thyroid tests in dogs with gastrointestinal disease.