Abstract: Thyroid hormones plasma concentrations values of 80 apparently healthy Iraqi sheep (40 ewes and 40 rams) were studied. The mean plasma T4 and T3 for ewes were 54.8±6.2 and 1.06±0.04 ng mL-1, respectively. The mean T4 and T3 were decreases as the age of rams progress, while in late pregnant ewes the mean T4 and T3 were significantly p<0.01 more than that of lactating and old ewes. The mean thyroid hormones value of both lactating and old ewes were, also significantly (p<0.01) different. The fetal thyroid gland weight were increased from 18-972.6 mg as fetal CRL increased from 5-45 cm. Histologically thyroid follicles containing colloid were seen at 20 cm CRL and follicles increased in number, size and colloid content as fetal CRL increased toward birth. The effect of age and sex of sheep produce different degree of variation in the thyroid hormones concentrations. The high levels of T4 and T3 in late pregnancy is essential for fetal growth and development. Fetal thyroid is functioning during gestation.