# Can thyroid medications affect the fetus during pregnancy? Canonical page: https://clinicsjo.com/en/faq/can-thyroid-medications-affect-fetus-during-pregnancy Language: en Category: pregnancy Publisher: ClinicsJo Published: 2026-09-13T17:52:53.390Z Content updated: 2026-09-13T17:52:53.390Z Translation (ar): https://clinicsjo.com/ar/faq/can-thyroid-medications-affect-fetus-during-pregnancy In general, thyroid medications are considered safe and necessary to maintain the health of both the mother and the fetus when used under careful medical supervision [\[4\]](). Thyroid hormones play a crucial role in the development of the fetal brain and nervous system, especially during the first trimester, when the fetus relies entirely on the mother's supply of these hormones [\[4\]](). Treatment aims to maintain balanced hormone levels to reduce the risk of complications such as miscarriage, preterm birth, or growth problems. ## Safety of Levothyroxine (Hypothyroidism Medication) for the Fetus Levothyroxine, a synthetic thyroid hormone, is the standard and safe treatment for hypothyroidism during pregnancy [\[4\]](). This medication is identical to the T4 hormone naturally produced by the thyroid gland, making it safe for the fetus. Due to the body's increased need for thyroid hormones during pregnancy, the mother may require an adjustment to her levothyroxine dosage. Doctors typically recommend increasing the dosage by two extra doses per week as soon as pregnancy is confirmed [\[5\]](). The physician should monitor thyroid hormone levels every 4 to 6 weeks during the first half of pregnancy, and at least once after 30 weeks, to adjust the dosage as needed [\[4\]](). It is very important to avoid medications containing T3 hormone extracted from animal glands, as these medications are not considered beneficial for fetal brain development and are not recommended during pregnancy [\[4\]](). ## Hyperthyroidism Medications and Their Effect on the Fetus In cases of hyperthyroidism, a doctor may prescribe anti-thyroid medications to reduce excess hormone production, preventing large amounts from reaching the fetus [\[4\]](). Propylthiouracil (PTU) is preferred during the first trimester of pregnancy because it is less likely to cause serious congenital defects compared to methimazole, although defects are rare with both medications [\[4\]](). Doctors may switch to methimazole after the first trimester because it is easier to use and has fewer side effects [\[4\]](). The lowest possible dose of these medications is prescribed to control hyperthyroidism in order to minimize the risk of the fetus developing hypothyroidism [\[4\]](). The mother must follow up with an endocrinologist or a maternal-fetal medicine specialist to ensure the correct dosage and careful monitoring of the fetus. ## Signs That Require Immediate Medical Attention Although thyroid medications are generally safe, you should be aware of any potential side effects. If you are taking anti-thyroid medications and experience any of the following symptoms, you must stop the medication and contact your doctor immediately, or go to the nearest emergency room if you cannot reach your doctor the same day: yellowing of the skin or whites of the eyes (jaundice), mild abdominal pain, persistent sore throat, or fever [\[4\]](). These symptoms may indicate rare but serious problems such as liver failure or a low white blood cell count [\[4\]](). You should also contact your doctor if new symptoms appear, such as increased fatigue or weakness, loss of appetite, rash or itching, or easy bruising [\[4\]](). For more information on thyroid disorders during pregnancy, you can visit our article on [Thyroid Disorders in Pregnancy: Diagnosis, Treatment, and Impact on Mother and Fetus (Arabic)](). ## Importance of Regular Medical Follow-up Regular follow-up with your doctor is essential to ensure you are receiving the correct dosage of thyroid medication throughout your pregnancy [\[4\]](). Your hormonal needs may change as the pregnancy progresses, and dose adjustments ensure that thyroid hormone levels are maintained within a healthy range, protecting your health and the health of your fetus. Do not adjust medication dosages yourself without medical consultation, as this may expose you and your fetus to health risks. ## Medical references - [Thyroid Disease & Pregnancy - NIDDK]() - [Hypothyroidism in Pregnancy | American Thyroid Association]() General health education; not a substitute for individualized clinical assessment. Citation URL: https://clinicsjo.com/en/faq/can-thyroid-medications-affect-fetus-during-pregnancy#answer