Hyperthyroidism during pregnancy

By trimester

Weeks 1–13

1st trimester

High hCG can temporarily raise thyroid hormone, especially with hyperemesis, and this usually fades as hCG falls. Graves disease is more likely with a prior thyroid history, goiter, eye findings, or thyroid-receptor antibodies. Propylthiouracil is generally preferred when an antithyroid drug is needed this early.

Weeks 14–27

2nd trimester

Persistent hyperthyroidism beyond the early hCG peak is less consistent with a transient gestational change. After 16 weeks, current guidance does not identify one preferred antithyroid drug. Continuing propylthiouracil or switching to methimazole requires an individualized specialist discussion and close thyroid testing.

Weeks 28–birth

3rd trimester

Graves activity may lessen later in pregnancy, so medicine needs can change. Ongoing maternal thyroid tests and, when antibody levels or disease severity warrant it, fetal growth, heart-rate, and thyroid surveillance help guide late-pregnancy planning.

What does the evidence show for hyperthyroidism?

• Graves disease is the usual cause of persistent hyperthyroidism in pregnancy, while high hCG can cause temporary hyperthyroidism early, particularly with severe vomiting. NIDDK describes thyroid hormone and antibody blood tests as part of distinguishing these patterns. 1
• Gestational transient thyrotoxicosis is hCG-mediated, usually mild, lacks the characteristic eye and goiter findings of Graves disease, and commonly resolves by about 15 weeks. Antithyroid drugs are not routinely indicated for this temporary pattern. 2
• The 2026 American Thyroid Association guideline prefers propylthiouracil when an antithyroid drug is needed in the first trimester. After 16 weeks, evidence does not establish whether continuing it or switching to methimazole is preferable. Continued medication requires close thyroid testing, generally every 2 to 4 weeks. 3
• A Danish registry cohort of 817,093 liveborn children found different birth-defect patterns associated with first-trimester methimazole or carbimazole and propylthiouracil exposure. The observational study used prescription and registry data, included live births, and could not remove confounding by maternal thyroid disease or establish causation. 4
• A systematic literature review found no single optimal management method across the published evidence. Antithyroid drugs were the main medical option, radioactive iodine was contraindicated, and thyroid surgery was reserved for selected severe cases, but the review was limited by heterogeneous studies and incomplete consensus. 5

When should I call my provider about hyperthyroidism?

Seek urgent care for a very fast or irregular heartbeat with chest pain, fainting, severe shortness of breath, confusion, high fever, or marked agitation because these can signal a thyroid or cardiac emergency. Persistent vomiting with inability to keep fluids down, reduced urination, weakness, or dizziness also needs prompt assessment, especially with known Graves disease or antithyroid medicine use. When in doubt, call your obstetric team the same day. Early input almost always beats waiting, and the on-call line exists exactly for this.