Thyroid cysts as evidence of past energy balance: a cross-sectional study

Introduction: Although the presence of thyroid cysts in dairy cows has been reported [1], the reason for their appearance remains a mystery [2]. The purpose of this study is to investigate the potential link in dairy cows between thyroid cysts (TC) observed on thyroid ultrasound (TU) [2] and their energy metabolism, assessed by blood assays and Body Condition Score (BCS). Animals, materials and methods. TU (Mindray, DP50-Vet; convex probe, 5 mHz) [1] and blood sampling were performed on 176 randomised milking cows from 5 different farms. Beta-hydroxybutyrate (BHB) and glucose were directly measured on fresh blood (Precision Xceed®, Abbott). In addition, urea, cholesterol, albumin, non-esterified fatty acids (NEFA), plasma inorganic iodine (PII), selenium, total thyroxine, insulin and leptin were assayed in 42 cows with TC and 43 healthy cows without TU anomalies. Healthy cows were defined via a clinical examination and blood test results. Results and discussion: Of the 176 cows, two groups were identified: healthy cows (n=42) and cows with one or more thyroid cysts (n=43; incidence 28%). The other 71 cows were excluded because they showed abnormalities in their clinical examination and/or blood tests. When comparing TC cows with healthy cows, the BCS of the first ones was significantly lower compared to healthy cows (p<0.01). PII and insulin were significantly lower in TC cows compared to healthy cows (p<0.05). An ordinal regression model revealed a significant interaction between insulin concentration and body condition score (β = −0.0067, t = -3.63). This result indicates that the association between insulin and thyroid cyst severity (β = 0.0133; t = 2.88) depends on the energetic status of the cow. Lower insulin concentrations appear particularly associated with cyst occurrence in animals with low body condition scores. Fig 1 shows that the probability of thyroid cysts was highest in cows with low body condition scores and low insulin concentrations. In contrast, cows with higher BCS showed a low predicted probability of cysts regardless of insulin levels. Principal component analysis did not identify a distinct metabolic profile associated with thyroid cyst presence. This likely reflects the fact that cyst formation represents the consequence of previous energetic imbalance rather than the current metabolic status measured at sampling. In human medicine, thyroid nodules (including cysts) are more common in women and associated with metabolic syndrome or metabolic stress [3,4]. This is the first time that the incidence of thyroid cysts and their link to metabolic parameters have been studied. The cross-sectional study design does not allow for the kinetics of thyroid cyst development to be studied. A longitudinal study monitoring thyroid cyst development in cows over one year would help clarify their pathophysiology and clinical significance. Conclusion: This study highlights an association between energy metabolism and the presence of thyroid cysts in cows. However, further studies are required to confirm this relationship and better understand the underlying mechanisms.