Thyroid Problems in Pregnancy: Underactive and Overactive Thyroid
Key message: Well-controlled thyroid disease usually allows a healthy pregnancy. Thyroid hormone requirements and antithyroid medicine choices can…
Key message: Well-controlled thyroid disease usually allows a healthy pregnancy. Thyroid hormone requirements and antithyroid medicine choices can change quickly, so preconception review, early blood tests and regular monitoring are important.
Get urgent help: Seek urgent care for severe palpitations with chest pain or breathlessness, fainting, high fever with agitation or vomiting, severe weakness, confusion or a rapidly enlarging neck with breathing difficulty. Do not stop thyroid medicine suddenly.
What does the thyroid do?
Thyroid hormones regulate metabolism and support fetal brain and nervous-system development. The fetus relies particularly on maternal thyroid hormone early in pregnancy.
Pregnancy changes normal test ranges, so results should be interpreted using pregnancy- and laboratory-specific thresholds.
What is hypothyroidism?
An underactive thyroid may cause fatigue, cold intolerance, constipation or weight change, but symptoms overlap with pregnancy. It is diagnosed using TSH and free T4, not symptoms alone.
Untreated significant hypothyroidism is associated with miscarriage, hypertension, preterm birth and developmental risk.
How is hypothyroidism treated?
- Levothyroxine is the standard replacement and is safe in pregnancy.
- A patient already taking levothyroxine should contact the clinician immediately after a positive test because the dose often needs increasing.
- TSH/free T4 are checked regularly, more frequently after dose changes.
- Iron, calcium and some antacids reduce levothyroxine absorption and should be separated.
- Desiccated thyroid or T3-only treatment is not routine pregnancy replacement.
What is hyperthyroidism?
Graves’ disease is a common cause. Symptoms can include tremor, heat intolerance, weight loss, anxiety and fast heartbeat, but hCG-related temporary thyroid change can also occur in early pregnancy or hyperemesis.
Uncontrolled disease raises risks including pre-eclampsia, preterm birth, fetal growth problems and thyroid storm.
How is hyperthyroidism treated?
- Antithyroid medicine at the lowest effective dose, selected according to gestation and individual risk.
- Propylthiouracil is often preferred early in pregnancy, with later review of whether to change medicine.
- Regular free T4/TSH and liver or blood-count review when clinically indicated.
- Radioactive iodine is contraindicated in pregnancy.
- Surgery is rarely needed and, when necessary, is usually planned in the second trimester.
Why are thyroid antibodies important?
TSH-receptor antibodies can cross the placenta in current or previous Graves’ disease, even after thyroid surgery or radioiodine. Levels help decide whether fetal heart rate, growth and thyroid signs require extra monitoring.
Thyroid peroxidase antibodies have different implications and do not by themselves determine all treatment.
What monitoring may the baby need?
- Routine anatomy and growth care for well-controlled hypothyroidism.
- Additional growth, fetal heart-rate, fluid or thyroid assessment in uncontrolled hyperthyroidism or significant receptor antibodies.
- Neonatal thyroid testing when maternal disease or antibodies require it.
- Medication and feeding plan after birth.
What happens after delivery?
Levothyroxine dose often returns toward the pre-pregnancy plan and is rechecked. Graves’ disease may flare postpartum, and postpartum thyroiditis can cause a temporary overactive then underactive phase.
Breastfeeding is compatible with levothyroxine and selected antithyroid doses under monitoring.
Frequently asked questions
Should every pregnant patient have a full thyroid panel?
Testing is based on symptoms, history and local antenatal policy rather than one universal panel.
Is levothyroxine safe?
Yes. It replaces a hormone the body needs.
Can I stop antithyroid medicine because I am pregnant?
No. Uncontrolled hyperthyroidism is dangerous; medicine should be adjusted by a specialist.
Does thyroid disease always cause infertility or miscarriage?
No. Risk is mainly increased when disease is untreated or poorly controlled.
Can thyroid medicine be taken with iron?
Separate levothyroxine from iron/calcium according to clinical advice because absorption is reduced.
Your next step
Arrange preconception or immediate early-pregnancy review with the exact medicine dose and recent TSH/free T4. Do not wait for the routine booking visit after a positive test.
Medical and content review: Dr Mohamed Abdelghany, Obstetrics & Gynecology Specialist, MRCOG (UK), MSc Obstetrics & Gynecology – Kasr Al-Ainy, Cairo University.
Last reviewed: 23 August 2026.
This information is for general education and does not replace individual medical assessment. Severe, sudden or concerning symptoms should not wait for a routine appointment.
