Thyroid Disorder During Pregnancy: Risks & Care Guide

What Is Thyroid Disorder During Pregnancy?

Thyroid disorder during pregnancy means the thyroid gland is producing too little thyroid hormone (hypothyroidism) or too much (hyperthyroidism) while a woman is pregnant. Because the fetus relies entirely on maternal thyroid hormone during the first trimester, before its own thyroid gland starts working, untreated thyroid disease in pregnancy can affect both maternal health and fetal brain development.

Thyroid disease is one of the more common endocrine conditions clinicians encounter in pregnancy. Some women already have a diagnosis before conceiving; others are diagnosed for the first time once pregnant, because pregnancy itself places new demands on the thyroid gland.

Why Pregnancy Changes Thyroid Function

Pregnancy hormones directly affect thyroid test results, which is why normal ranges look different than they do outside of pregnancy. The American Thyroid Association explains that human chorionic gonadotropin — the hormone that pregnancy tests detect — can weakly stimulate the thyroid gland, which is why thyroid-stimulating hormone often runs lower than usual in the first trimester before rising again later in pregnancy. Estrogen also increases thyroid-hormone-binding proteins in the blood, raising total thyroid hormone levels even though the "free," active hormone usually stays normal.

Because of these shifts, doctors interpret thyroid blood work using pregnancy- and trimester-specific reference ranges rather than standard non-pregnant ranges whenever they're available.

[REVIEWER: add clinical insight here — for example, how you explain trimester-specific TSH ranges to a patient who is used to seeing her "normal" non-pregnant thyroid numbers]

Hypothyroidism in Pregnancy

Hypothyroidism in pregnancy means an underactive thyroid gland isn't making enough hormone to meet the body's higher demand. The American Thyroid Association notes that a TSH level above roughly 4 mIU/L, or above the local trimester-specific reference range, is generally used to define hypothyroidism when a pregnancy-specific range isn't available.

Women already diagnosed with hypothyroidism before pregnancy typically need to increase their levothyroxine dose as soon as pregnancy is confirmed, since hormone requirements rise during gestation. According to the ATA, women with a TSH above 10 in the first trimester should be treated, while thyroid function testing should generally continue roughly every four weeks, or with any dose change, to keep levels in the target range.

Autoimmune thyroid disease also plays a role: research cited in the ATA's guidelines found that a meaningful share of women who test positive for thyroid antibodies but start pregnancy with normal thyroid function go on to develop an elevated TSH as pregnancy progresses, because the added demand outpaces what an already-stressed thyroid gland can produce.

Hyperthyroidism in Pregnancy

An overactive thyroid in pregnancy is less common than hypothyroidism but carries its own risks, including links to preterm birth and, in severe cases, maternal heart complications. Graves' disease — an autoimmune condition — is the most frequent underlying cause outside of the mild, temporary hCG-related TSH suppression seen in early pregnancy.

The American College of Obstetricians and Gynecologists recommends that pregnant women with confirmed overt hyperthyroidism be treated with anti-thyroid medication (a thioamide), with free T4 levels monitored regularly so the dose can be adjusted as pregnancy progresses. Because some anti-thyroid medications carry different risk profiles at different points in pregnancy, the specific drug and dose is a decision made individually with a treating physician.

[REVIEWER: add clinical insight here — for example, how you counsel a patient recently diagnosed with Graves' disease who is planning a pregnancy about timing and medication choice]

Who Gets Tested, and When

Universal thyroid screening for every pregnant woman is not currently recommended by ACOG. Instead, testing is generally risk-based, targeting women with a personal or family history of thyroid disease, autoimmune disease, infertility or recurrent pregnancy loss, or use of medications known to affect thyroid function.

The ATA takes a similar risk-based approach but recommends checking TSH as soon as pregnancy is confirmed in higher-risk women — including those previously treated for hyper- or hypothyroidism, those with a family history of thyroid disease, or those with a visibly enlarged thyroid gland (goiter). In 2026, the ATA published updated guidelines on thyroid disease across preconception, pregnancy, and postpartum, refining recommendations on testing, iodine intake, and management of thyroid autoimmunity based on evidence gathered since the prior 2017 guidelines.

Iodine Needs During Pregnancy

Because the fetus depends on maternal iodine to make its own thyroid hormone later in pregnancy, iodine needs increase substantially. The NIH's Office of Dietary Supplements recommends 220 micrograms of iodine daily during pregnancy, compared with 150 micrograms for non-pregnant adults, typically met through a combination of diet and a prenatal vitamin containing iodine.

After the Baby Is Born: Postpartum Thyroiditis

Some women develop postpartum thyroiditis, a temporary inflammation of the thyroid gland that can cause a brief period of hyperthyroidism, hypothyroidism, or both in sequence during the months after delivery. It's more common in women with pre-existing thyroid antibodies. Thyroid function typically needs monitoring in the months following delivery for any woman treated for thyroid disease during pregnancy, since medication doses that were right during pregnancy are often too high once hormone demands drop back down.

The Bottom Line

Thyroid disorder during pregnancy is common, testable, and treatable — the key is catching it early enough to protect fetal brain development, especially in the first trimester before the baby's own thyroid gland is functioning. Women with a personal or family history of thyroid disease, autoimmune conditions, or prior pregnancy complications should ask their obstetric provider about thyroid testing as soon as pregnancy is confirmed, and anyone already on thyroid medication should expect their dose to be reassessed throughout pregnancy and again after delivery.

Frequently Asked Questions

Can untreated thyroid disease during pregnancy harm the baby? Yes. Because the fetus depends on maternal thyroid hormone early in pregnancy, untreated maternal hypothyroidism or hyperthyroidism has been linked to pregnancy complications and effects on fetal neurological development, which is why testing and treatment matter, particularly in the first trimester.

Is it safe to take thyroid medication while pregnant? Levothyroxine, used for hypothyroidism, is considered standard, necessary treatment during pregnancy, and doses are typically increased to meet higher demand. Anti-thyroid medications for hyperthyroidism are also used in pregnancy, with the specific drug and dose chosen individually based on the trimester and the woman's history.

Why does TSH need to be checked differently during pregnancy? Pregnancy hormones, especially , temporarily change normal TSH levels, particularly in the first trimester. Doctors use pregnancy- and trimester-specific reference ranges instead of standard non-pregnant ranges to avoid over- or under-diagnosing thyroid disease.

Does every pregnant woman need a thyroid test? No. Universal screening isn't currently recommended by . Testing is generally offered to women with risk factors such as a personal or family history of thyroid disease, autoimmune conditions, or infertility, though guidelines continue to be studied and updated.

What is postpartum thyroiditis? Postpartum thyroiditis is a temporary thyroid inflammation that can occur in the months after delivery, sometimes causing a brief period of an overactive thyroid followed by an underactive one. It's more common in women with pre-existing thyroid antibodies and usually resolves, though some women go on to develop permanent hypothyroidism