Thyroid Disease and Pregnancy: Risks and What to Know

Why does the thyroid matter so much during pregnancy?

The thyroid gland produces hormones — primarily T4 (thyroxine) and T3 (triiodothyronine) — that regulate metabolism throughout the body. During pregnancy, thyroid hormone demand increases significantly for two reasons: the growing placenta and fetus, and a pregnancy hormone called hCG that stimulates the thyroid. A thyroid that is already struggling may not keep pace. Crucially, the fetal brain depends on maternal thyroid hormone for development during the first trimester, before the fetal thyroid begins functioning on its own.

How does hypothyroidism affect pregnancy?

Hypothyroidism — where the thyroid produces too little hormone — is associated with an increased risk of:

• Miscarriage and recurrent pregnancy loss
• Preterm delivery
• Preeclampsia (a serious pregnancy complication involving high blood pressure)
• Placental abruption
• Low birth weight
• Impaired fetal neurodevelopment, particularly when severe and untreated
• Gestational hypertension

The most common cause of hypothyroidism in pregnancy is Hashimoto's thyroiditis, an autoimmune condition in which the immune system attacks the thyroid gland 1Ref 1Weetman AP (2021).An update on the pathogenesis of Hashimoto's thyroiditis.Hashimoto's thyroiditis as most common cause of hypothyroidism in pregnancy2Ref 2National Institute of Diabetes and Digestive and Kidney Diseases (2021).Hashimoto's Disease.Autoimmune thyroid disease association with miscarriage and recurrent pregnancy loss.

Subclinical hypothyroidism — where TSH is mildly elevated but T4 is still in the normal range — has a less clear-cut risk profile, and decisions about whether to treat are individualized, often depending on whether thyroid antibodies are present 3Ref 3Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.TSH targets during pregnancy and levothyroxine dosing adjustments; monitoring schedule.

How does hyperthyroidism affect pregnancy?

Hyperthyroidism — too much thyroid hormone — also carries risks if uncontrolled:

• Miscarriage
• Preterm birth
• Fetal growth restriction
• Thyroid storm (a rare but serious crisis of extreme overactivity, requiring emergency care)
• Neonatal hyperthyroidism — if the mother has Graves' disease, antibodies can cross the placenta and stimulate the fetal thyroid

Graves' disease is the most common cause of true hyperthyroidism in pregnancy 4Ref 4Ross DS, Burch HB, Cooper DS, Greenlee MC, Laurberg P, Maia AL, Rivkees SA, Samuels M, Sosa JA, Stan MN, Walter MA (2016).2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis.Graves' disease as main cause of hyperthyroidism in pregnancy; PTU vs methimazole guidance and contraindication of radioactive iodine. Transient gestational hyperthyroidism driven by hCG — often accompanying severe nausea and vomiting — is a different condition that usually resolves without antithyroid medication.

Can thyroid disease cause miscarriage?

Yes, both overt hypothyroidism and the presence of thyroid autoantibodies (even with normal thyroid levels) are associated with higher rates of miscarriage and recurrent pregnancy loss. The mechanism is not fully understood but likely involves inflammatory processes and impaired uterine receptivity. Testing for thyroid antibodies — particularly anti-thyroid peroxidase (anti-TPO) antibodies — is often considered when evaluating recurrent pregnancy loss 2Ref 2National Institute of Diabetes and Digestive and Kidney Diseases (2021).Hashimoto's Disease.Autoimmune thyroid disease association with miscarriage and recurrent pregnancy loss.

What is the normal TSH range during pregnancy?

TSH targets in pregnancy are lower than outside of pregnancy because hCG suppresses TSH naturally during the first trimester. Professional guidelines generally recommend trimester-specific TSH targets — roughly:

• First trimester: 0.1–2.5 mIU/L (some guidelines use up to 3.0 mIU/L)
• Second trimester: 0.2–3.0 mIU/L
• Third trimester: 0.3–3.0 mIU/L

These are reference ranges, not absolute cutoffs — your clinician will interpret your result in the context of your laboratory's specific population-based reference intervals and whether antibodies are present 3Ref 3Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.TSH targets during pregnancy and levothyroxine dosing adjustments; monitoring schedule.

How is thyroid disease treated during pregnancy?

Hypothyroidism: The standard treatment is levothyroxine (synthetic T4). Women who are already on levothyroxine typically need a dose increase early in pregnancy — sometimes as soon as pregnancy is confirmed. Dose is adjusted based on TSH monitoring, usually every four weeks in the first half of pregnancy, then every four to six weeks thereafter 3Ref 3Jonklaas J, Bianco AC, Bauer AJ, et al. (2014).Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement.TSH targets during pregnancy and levothyroxine dosing adjustments; monitoring schedule.

Hyperthyroidism: The antithyroid medications propylthiouracil (PTU) and methimazole are used, with PTU generally preferred in the first trimester due to concerns about methimazole's teratogenic effects; methimazole is often switched in the second trimester. Radioactive iodine is contraindicated in pregnancy. Surgery is occasionally needed in the second trimester when medications are not tolerated 4Ref 4Ross DS, Burch HB, Cooper DS, Greenlee MC, Laurberg P, Maia AL, Rivkees SA, Samuels M, Sosa JA, Stan MN, Walter MA (2016).2016 American Thyroid Association Guidelines for Diagnosis and Management of Hyperthyroidism and Other Causes of Thyrotoxicosis.Graves' disease as main cause of hyperthyroidism in pregnancy; PTU vs methimazole guidance and contraindication of radioactive iodine.

Who should be screened for thyroid problems in pregnancy?

Universal screening of all pregnant people is debated; targeted case-finding is more commonly practiced. Screening is recommended for people with symptoms of thyroid disease, a personal or family history of thyroid disorder or autoimmune disease, prior thyroid surgery or radioactive iodine therapy, previous preterm birth or miscarriage, type 1 diabetes or other autoimmune conditions, a goiter, or who live in an iodine-deficient region. If you fall into any of these groups and are planning a pregnancy or are newly pregnant, a conversation with your clinician about thyroid testing makes sense.

How Gale can help

Thyroid disease in pregnancy is managed by a team — often your obstetrician and an endocrinologist working together. Gale can help you understand your lab results, prepare questions for your appointments, and flag anything that needs prompt attention. If you have a known thyroid condition and are planning a pregnancy, connecting with your clinician before conception to optimize your TSH is one of the most effective steps you can take.