Thyroid hormones (T3 and T4) serum levels were evaluated in crossbred zebu gestating cows and in their foetuses. The cows were raised in hot, low land and were alloted to a pregnancy groups according to their age of gestation (1th, 2nd or 3th trimester) determinated by crown-rump length o the foetuses. Mean values of T3 and T4 were 2.13 and 102.76 nmol/L respectively. There were no significant differences among the three gestation periods. T3 levels were lower in foetuses than in cows for comparable stages of gestation (average 1.11 nmol/L) while levels of T4 values were greater (average 153.9 nmol/L), In foetuses thyroid hormone levels increased with the gestation length and were significantly in the last trimester. The other hand, the thyroid function in the dam is totally independent the her foetus.