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This research was carried out to study thyroid function in view of serum trace element and possible serum cortisol concentration changes in crossbred Holstein cattle naturally infected with Theileria annulata. Twenty cattle of different age and sex which had been naturally infected with T. annulata were examined. A control group were selected among the clinically healthy crossbred Holstein cattle with the same age, sex, and production stage similar to the naturally infected group. Serum T3, T4 concentrations were significantly lower in cattle suffering from theileriosis than in the healthy controls (p < 0.05). The cattle suffering from theileriosis had significantly lower concentrations of zinc and selenium in their sera as compared with the healthy control subjects (p < 0.05). Lower packed cell volume (PCV) theileriosis-affected cattle had a lower serum selenium concentration than higher PCV-affected group (p < 0.05). Significant differences were also found in PCV, total protein, albumin and total bilirubin concentrations and alkaline phosphatase and gama-gluthamyl transferase activities between theileriosis-affected and healthy cattle (p < 0.05). Total protein and albumin concentrations were statistically different in age (less and more than two years) and PCV (lower and higher) groups of theileriosis-affected cattle (p < 0.05). No significant changes were found in free T3, free T4, cortisol, copper, cobalt, and manganese levels of theileriosis-affected cattle.