Thyrotropin in newborn calves as a tool for diagnosing hypothyroidism

The hypothalamic-pituitary-thyroid axis is of particular importance for the adaptation of mammals to their environment. Hypothyroidism is the most common type of thyroid disorders in cattle. Thyrotropin (TSH) is used in several mammalian species for the diagnosis of primary hypothyroidism. Currently in cattle, thyroid hormone assays are used to diagnose such disease. The aim of the study was to compare different tools for diagnosing hypothyroidism in newborn calves. Twelve goitrous newborn calves from two beef herds were studied as well as 45 healthy newborn calves. Bovine TSH, thyroxine (T4), tri-iodothyronine (T3), reverse-T3 (rT3), plasmatic inorganic iodine (PII) and glutathione peroxidase in erythrocytes (GPXe) were assayed in these calves during the first day of life. T4/T3 and T4/TSH ratio were calculated. Furthermore, TSH, T4, PII and GPXe were also assayed in their dams at calving. Results were analysed using Wilcoxon rank sum test and Spearman's rank correlation coefficient. A significant higher value (P < 0.01) of TSH was seen for the group of goitrous calves compared to healthy calves. There was no difference between healthy and goitrous calves for GPXe (P > 0.1) and PII (P > 0.05). Significantly higher values were seen for healthy calves compared to goitrous calves concerning T4, T4/T3 ratio, T4/TSH ratio, rT3 (P < 0.0 1) and T3 (P < 0.05). Regarding the group of goitrous calves, those that had larger goitre at palpation and were hairless (n=8) died within the first day of life while the four other with moderate goitre and normal hair stayed alive. Goitrous-dead calves had higher TSH values than goitrous-alive ones (P < 0.01), lower T4, T3, T4/TSH (P < 0.0 1) and rT3 (P < 0.05) but similar T4/T3 ratio (P > 0.1). Furthermore, it appeared that T4 and rT3 values in goitrous-alive calves were not different compared to healthy calves (P > 0.1). Regarding TSH (rho=0.44), T4 (rho=-0.44), PII (rho=0.70) and GPXe (rho=0.87), there was a correlation between healthy calves and their dams, with significantly higher values being found in the calves (P < 0.01). There was no correlation between values of TSH and T4 in goitrous calves and their dams (P > 0.1). However, for these animals there was a strong relationship in the values of PH (rho=0.82) and GPXe (rho=0.94) (P < 0.01). TSH, PII and GPXe were significantly higher in goitrous calves compared to their dams (P < 0.01). Regarding T4, only goitrous-dead calves showed significantly lower values than their dams (P < 0.01). Dams with goitrous calves had higher TSH (P < 0.01) and PII (P < 0.05) compared to dams with healthy calves but similar T4 and GPXe (P > 0.1). This study revealed that TSH alone is a good marker for hypothyroidism in newborn calves. In the absence of TSH assay, the T4/T3 ratio may be used to diagnose hypothyroidism in newborn calves. In our study, although T4/T3 ratio was helpful in identifying goitrous calves, it did not allow to discriminate goitrous-dead from goitrous-alive calves, contrary to TSH. In our study, the presence of a goitre in newborn calves could not be explained by the iodine (PII) and thyroid (via T4) status of their dams.