The present investigation was performed to determine the effect of long-term administration of sodium selenite on the biochemical indices and concentration of thyroid hormones in calves. Sodium selenite solution was drenched at 0.1 and 0.25 mg per kg body weight for 12 weeks. Higher dose produced characteristic symptoms of selenosis whereas mild symptoms were observed with lower dose. The toxic symptoms appeared when blood selenium level was 1.68 ± 0.13 mg per ml. There was a significant (P<0.05) increase in the activities of the aspartate aminotransferase, alkaline phosphatase, blood urea nitrogen and creatinine levels in the treated animals. The repeated administration of sodium selenite resulted in a significant p-value decline in thyroxine levels on the 10th week and increase in triiodothyronine on the 8th week of treatment. The findings of the present study suggested that sodium selenite induced selenosis alters thyroid hormone levels in plasma.