BACKGROUND: Thyroid dysfunction can cause adverse effects on metabolic changes in the body and disturbances during the gestational period and intrauterine development of the fetus. In developing prevention methods, it is crucial to objectively substantiate and re-confirm theoretical assumptions. AIM: To assess the functional state of the fetus in pregnant women with certain thyroid diseases. MATERIAL AND METHODS: The results of instrumental studies in 118 pregnant women, who were divided into three clinical groups, were analyzed. Group 1 (44 people) consisted of patients with autoimmune thyroiditis (AIT). Group 2 (52 people) included patients with diffuse toxic goiter. Group 3 (control group) consisted of 22 pregnant women who had no somatic pathology and gestational complications. Thyroid diseases were diagnosed by an endocrinologist during a consultation in the third trimester of pregnancy based on laboratory and instrumental data. The functional state of the fetus was assessed using the total result of cardiotocographic and ultrasound studies, for which Partecust (Siemens, Germany) and Aloka-1700 (Hitachi, Japan) were used. Statistical processing of clinical material was performed using StatSoft software (Russia). RESULTS: A comprehensive assessment of the adaptive capabilities of the fetus showed changes in its functional state in pregnant women with thyroid pathology. Prognostically, the most crucial characteristic in pregnant women with autoimmune thyroiditis and diffuse toxic goiter is a decrease to 0–1 point in indicators that determine motor activity and tone, respiratory movements, and reaction to a non-stress test. Among patients with autoimmune thyroiditis and diffuse toxic goiter, a high assessment of the biophysical profile of the fetus occurred 4 and 3.4 times, respectively, less often than among women with a normal pregnancy (among pregnant women without somatic pathology and gestational complications). CONCLUSION: Disturbances in thyroid gland function during the gestational period can have an adverse effect on the development of the fetus, which is a theoretical basis for the development of treatment and preventive methods aimed at improving perinatal outcomes in this group of patients.