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Hypothyroidism & Pregnancy: What You Need To Know

Laura Tennant

Medically reviewed by


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If you have a thyroid condition such as hypothyroidism, and you are pregnant or considering having children, you may be wondering how hypothyroidism affects pregnancy. 

Hypothyroidism is relatively common, and affects between 3-5% of pregnant women. Untreated hypothyroidism (underactive thyroid) and subclinical hypothyroidism (slightly underactive thyroid) during pregnancy are associated with negative outcomes for the baby, including an elevated risk of pregnancy complications, and even lower IQ of the child.

If hypothyroidism is properly treated with medication, however, you can have a healthy and normal pregnancy. 

What is hypothyroidism?

Hypothyroidism is a medical condition where the thyroid does not produce sufficient levels of hormones. Thyroid hormones play a key role in maintaining homeostasis, or balance, in the body. They are responsible for regulating the body’s metabolism and energy breakdown, body temperature, and heart rate.

When a person has hypothyroidism, their body does not produce enough thyroid hormones. The condition must be treated with medication to replace the low levels of hormones. 

Symptoms of hypothyroidism:

  • Weight gain
  • Fatigue
  • Hair loss
  • Constipation
  • Dry skin

Importantly, many of the symptoms of hypothyroidism are similar to symptoms of pregnancy. This means it can be difficult to tell if you have hypothyroidism during pregnancy. The best way to find out is to have your thyroid levels tested.

Subclinical hypothyroidism

Subclinical hypothyroidism is when a person has a slightly underactive thyroid, and therefore slightly lower levels of thyroid hormones than would be ideal. However, subclinical means it is below the threshold that requires treatment. Subclinical hypothyroidism is relatively common, occuring in 3-8% of the general population

During pregnancy, however, subclinical hypothyroidism may present a risk to the baby. Speak to your healthcare provider if you are concerned about your thyroid.

Risk factors for maternal hypothyroidism

There are several factors that may increase your risk of hypothyroidism during pregnancy. You may be at increased risk if:

  • You are currently being treated for a thyroid condition
  • You have a family history of autoimmune thyroid conditions
  • You have or have had a goiter (swollen thyroid)
  • You have had high-dose radiation of the neck
  • You have type 1 diabetes
  • You are iodine deficient  

How Hypothyroidism can affect pregnancy: The science 

Studies suggest that hypothyroidism and subclinical hypothyroidism are both associated with worse outcomes for infants. However, when the condition is properly treated, the risks are lessened.  

Untreated hypothyroidism in pregnancy is associated with very high rates of complication.

A 1988 study published in Obstetrics and Gynecology found that women with untreated hypothyroidism frequently had complications in pregnancy, including “anemia (31%), preeclampsia (44%), placental abruption (19%), [and] postpartum hemorrhage (19%)”. Their babies were also more likely to die soon after birth. 

The authors noted that full-blown hypothyroidism during pregnancy was uncommon, because women with untreated hypothyroidism frequently do not ovulate. 

Subclinical hypothyroidism during pregnancy is also associated with higher rates of complication. 

A 2005 study published in Obstetrics and Gynecology followed 25,756 women through pregnancy. Of women who enrolled for prenatal care, 2.3% of these were identified as having subclinical hypothyroidism.

Compared to controls without subclinical hypothyroidism, pregnancies in the group with subclinical hypothyroidism were 3 times more likely to have a complication called placental abruption. They were also twice as likely to have a preterm birth.

After birth, the number of infants admitted to the NICU, as well as the number of infants developing respiratory distress syndrome were both twice as high in the subclinical hypothyroid group compared with controls. This is likely related to the risks of preterm birth. 

Babies born to mothers with subclinical hypothyroidism had significantly lower IQs at age 7 or 8.

A 2015 case-control study found that children aged 7 or 8 whose mothers had subclinical hypothyroidism during pregnancy had a significantly lower IQ score than controls whose mothers did not have subclinical hypothyroidism. The hypothyroid group’s average IQ score was 103.87, while the control group’s average score was 109.11. 

The authors suggest routine prenatal screening for subclinical hypothyroidism. However, a 2013 review published in The Lancet suggests the opposite: “...because benefits of therapy are still uncertain, universal screening of all pregnant women for subclinical hypothyroidism or thyroid autoimmunity is not recommended by most professional groups.” 

Children of women with low T4 during early pregnancy had impaired psychomotor development. 

A 1999 study found that children whose mothers’ T4 levels were low had significantly impaired psychomotor development at age 10 months. T4 is a thyroid hormone that is lower in people with hypothyroidism. 

The authors conclude that low maternal T4 levels during early pregnancy could be a risk factor for impaired child development.  

Early screening and treatment for subclinical hypothyroidism during pregnancy did not significantly change IQ scores of children at age 3. 

A 2012 study screened pregnant women for subclinical hypothyroidism and found that treatment did not affect the IQ score of the child at age 3. The IQ scores between the treatment and control group were not significantly different. 

These findings show that treating subclinical hypothyroidism did not affect IQ score. However, the study is limited because researchers did not compare the group to children of mothers who did not have subclinical hypothyroidism. 

Treating hypothyroidism during pregnancy

Hypothyroidism is normally treated with medication called levothyroxine, which is a synthetic form of thyroid hormone. It can help compensate for an underactive thyroid. During pregnancy, a doctor may prescribe this medication to treat hypothyroidism. 


Your physician will most often advise you to resolve any health concerns before pregnancy, if possible. Ideally, hypothyroidism should be treated before conception. If you are planning a pregnancy, and have symptoms of hypothyroidism, speak to your doctor before conceiving.

During pregnancy

Treating hypothyroidism during pregnancy is not as straightforward as it is when a person is not pregnant. Pregnancy can increase the amount of thyroid hormone the body needs which can often result in a doctor prescribing a higher dose of levothyroxine. More than 50% of women with hypothyroidism will need their medication increased when they become pregnant.

Pregnant women with hypothyroidism require regular tests (usually between 4-6 weeks apart) to ensure their medication is sufficient to bring their T4 levels down to support a healthy pregnancy. 


During the postpartum period, the dose of levothyroxine can generally be reduced to the pre-pregnancy dosage.

Some women may experience postpartum thyroiditis, an autoimmune response that causes an inflamed thyroid gland during the first few months postpartum. This can cause hypothyroidism, but it usually resolves over time. Postpartum thyroiditis typically does not require treatment with levothyroxine


If you take medication for hypothyroidism and are planning on breastfeeding, it is a good idea to have a discussion with your healthcare provider. Generally, thyroid medications are considered safe for breastfeeding. Your doctor may adjust your dose once you are no longer pregnant. 

Subclinical hypothyroidism

When it comes to treating subclinical hypothyroidism, there are conflicting reports. Some studies indicate that there is no difference in preterm delivery or birthweight between the treated and untreated subclinical hypothyroid groups. Another study found that there was a decrease in adverse pregnancy outcomes among women with subclinical hypothyroidism treated with levothyroxine. If you are concerned that you could have subclinical hypothyroidism, it’s a good idea to speak to your healthcare provider about whether treatment is necessary.

The bottom line

Hypothyroidism during pregnancy is linked to many adverse outcomes for moms and babies, including preterm labour, placental abruption, and other pregnancy complications. Subclinical hypothyroidism during pregnancy, which can go undetected, may be linked to lower IQ scores in babies, as well as pregnancy complications. 

If you have symptoms of hypothyroidism and are pregnant or planning a pregnancy, it’s a good idea to speak to your healthcare provider. Your healthcare provider may advise you to delay pregnancy until the disease is under control. You may need to take medication, or have your dose changed while you are pregnant. Importantly, you can have a healthy pregnancy if your condition is treated properly.

Updated on
October 21, 2021
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