The thyroid is a small gland in your neck that helps regulate energy, metabolism, heart function, and brain activity. During pregnancy, thyroid hormone also plays a direct role in fetal growth and development, especially in the first trimester, when your baby relies heavily on your hormone supply.
When thyroid levels are too high or too low, it can affect how you feel and, in some cases, increase pregnancy risk. The good news is that thyroid disease is one of the more manageable conditions in pregnancy when it is caught early and monitored consistently.
Why Thyroid Levels Shift During Pregnancy
Pregnancy changes nearly every hormone system in your body, and the thyroid is no exception. Several factors drive these shifts.
First, pregnancy increases a blood protein that binds to the thyroid hormone. When more of the hormone is bound, less is freely available for your body to use, which can affect both lab values and medication needs. Second, early pregnancy produces high levels of hCG (human chorionic gonadotropin)—the so-called “pregnancy hormone,” which can stimulate the thyroid and temporarily lower thyroid stimulating hormone (TSH) levels, especially in the first trimester. Third, as pregnancy progresses, your placenta and growing baby increase your overall hormone demands.
Because of these changes, thyroid monitoring is not a one-time event. It is a process that evolves across each trimester.
Hypothyroidism in Pregnancy
Hypothyroidism means your thyroid is underactive and does not produce enough hormone. One of the most common causes is Hashimoto’s thyroiditis, an autoimmune condition. Some women enter pregnancy with a known diagnosis, while others are identified during pregnancy through routine lab work.
Symptoms can include fatigue that feels more intense than typical pregnancy tiredness, constipation, feeling cold, dry skin, hair changes, and brain fog. Because many of these overlap with normal pregnancy, lab testing is the most reliable way to assess thyroid function.
When hypothyroidism is untreated or poorly controlled, it can increase the risk of miscarriage, preeclampsia, anemia, preterm birth, and in more severe cases, effects on fetal development. That does not mean complications are expected. It means consistent thyroid management is a meaningful form of prevention.
The most common treatment for hypothyroidism is thyroid hormone replacement, typically levothyroxine. Many women need a higher dose during pregnancy, which reflects normal physiology and an increased demand for hormones rather than a worsening condition. One practical note: prenatal vitamins often contain iron and calcium, which can interfere with thyroid medication absorption. Separating your thyroid dose from your prenatal vitamin by several hours can help.
Hyperthyroidism in Pregnancy
Hyperthyroidism is a condition in which the thyroid is overactive and produces too much hormone. The most common cause of hyperthyroidism during pregnancy is Graves’ disease, which is also an autoimmune disease. Some women are diagnosed before pregnancy, while others develop symptoms during pregnancy that lead to evaluation and diagnosis.
Symptoms can include rapid heartbeat, feeling shaky, heat intolerance, trouble sleeping, unexplained weight loss or poor weight gain, and anxiety that feels out of proportion. While some nausea and vomiting in early pregnancy is normal, severe symptoms combined with a racing heart and significant weight changes may prompt thyroid testing.
When hyperthyroidism is uncontrolled, it can increase risks such as pregnancy-related hypertension, preterm birth, fetal growth issues, and in certain Graves’ disease cases, fetal or newborn thyroid concerns. Treatment depends on the cause and severity of symptoms.
Some mild hyperthyroidism cases related to early pregnancy hormone shifts improve on their own. Graves’ disease may require medication, and dosing is typically kept at the lowest effective amount to support maternal health while minimizing fetal exposure.
