In this investigation, 24 Swiss Brown Cows which were clinically healthy and similar age were used During the two month, the blood samples were taken from dry cows and early, mild an late lactalion animals at 15 days intervals Total thyroxine (TT₄), total triiodothyronine (TT₃), glucose and total cholesterol levels In the blood serum were determined Daily milk production ot the animals in the lactation stages were measured The mean daily milk production in the early, mild and late lactation cows were found to be 13.94 lt, 10.56 It and 8.69 It respectively. It was determined significantly differences among the mean blood serum TT₄ (4.19 µg/dl, 4.19 µg/dl, 4.50 µg/dl and 4 64 µg/dl respectively) TT₃ (95.67 ng/dl, 106.89 ng/dl. 110, 82 ng/dl and 106 10 ng/dl respectively) glucose (57.94 mg/dl, 58.08 mg/dl, 62 41 mg/dl and 64 35 mg/dl respectively) and total cholesterol (94.34 mg/dl, 93.66 mg/dl, 88.46 mg/dl and 86,11 mg/dl respectively levels of dry animals and cows in the early, mild and late lactation stages. Based on results, it was concluded that lactation was significantly affected the levels of the blood serum TT₄, TT₃, glucose and total cholesterol depend on daily milk production in the cows.