For patients with a history of hypothyroidism who are planning to become pregnant, it is necessary to reduce thyroxine levels and achieve an increase in serum TSH to <2.5 µIU/mL. This adjustment helps reduce the risk of elevated TSH levels during the first trimester of pregnancy. During pregnancy, estrogen levels and the concentration of thyroid-binding globulin increase, which leads to a rise in T4 and T3 hormone levels. In the first trimester, serum TSH levels decrease under the influence of human chorionic gonadotropin (hCG), which is associated with a slight and temporary increase in free T4 (FT4). These changes are usually minor, and in most pregnant women, FT4 concentrations remain within the normal range seen in non-pregnant women. In the second and third trimesters, FT4 and FT3 levels tend to decrease, falling below the standard levels seen in non-pregnant women. There is insufficient evidence to fully support or oppose universal thyroid screening during pregnancy. Screening does not significantly reduce the risk of adverse outcomes, even when compared to the identification and treatment of thyroid hormone dysfunction. However, the Endocrine Society recommends targeted screening for women at risk of thyroid dysfunction. These include: • Women over the age of 30 • Women on levothyroxine replacement therapy • Women with iodine deficiency living in endemic areas • Women with a history of hyperthyroidism, hypothyroidism, postpartum thyroiditis, or thyroid lobectomy • Women with a family history of thyroid disease • Women who test positive for thyroid autoantibodies (if known) • Women with symptoms or clinical signs of thyroid dysfunction (e.g., anemia, high cholesterol, or hyponatremia) • Women with type 1 diabetes • Women with other autoimmune diseases • Women with a history of head or neck radiation • Women with a history of preterm delivery or miscarriage • Women with infertility, as part of the clinical work-up for infertility, TSH levels should be assessed Pregnant women with hypothyroidism should have their hormone levels closely monitored, as in most cases, thyroxine dosage needs to be increased by 30–50% during the first trimester. Recommended TSH level targets by trimester: • First trimester: 0.1–2.5 µIU/mL • Second trimester: 0.2–3.0 µIU/mL • Third trimester: 0.3–3.0 µIU/mL Thyroxine dosage should be adjusted accordingly to maintain these levels. TSH levels should be monitored every 4 weeks until the 20th week of pregnancy, and then again at 26 and 32 weeks. After delivery, the thyroxine dose should be reduced to the pre-pregnancy level, and TSH should be retested after 6 weeks postpartum to determine if further dosage adjustment is necessary.