Subclinical hypothyroidism is a mild form of hypothyroidism. It occurs when only one of two main thyroid levels is elevated. It doesn’t usually cause symptoms, and it’s unclear how or if it affects you or your baby.

pregnant woman discussing subclinical hypothyroidism with her doctorShare on Pinterest
SDI Productions/Getty Images

Subclinical hypothyroidism in pregnancy happens when thyroid blood tests shows that you have elevated thyroid stimulating hormone (TSH) levels but normal free thyroxine (T4) levels. The vcondition can be common in pregnancy and happens more often than full hypothyroidism in pregnancy.

Subclinical hypothyroidism affects about 4–8% of individuals of reproductive age.

Research on the impact of subclinical hypothyroidism on pregnancy outcomes varies. Some studies suggest there are pregnancy risks, while others find no such risk.

Read on for what to know about subclinical hypothyroidism, including causes, risk factors, treatment, complications, and outlook.

Learn more about subclinical hypothyroidism.

Subclinical hypothyroidism during pregnancy is when your T4 (free thyroxine) levels are normal, but your TSH (thyroid stimulating hormone) levels are elevated. This is usually discovered during a blood test in early pregnancy.

Research from 2017 on the impact of subclinical hypothyroidism during pregnancy is mixed, with some studies finding an association between subclinical hypothyroidism and complications like miscarriage, gestational diabetes, and low birth weight, and other studies finding low or no risk.

Subclinical hypothyroidism is more common during pregnancy than overt hypothyroidism and is also less serious.

Hypothyroidism has been shown to increase the risk of miscarriage, high blood pressure, preterm birth, placental abruption, C-section birth, and postpartum hemorrhaging.

For this reason, if you have hypothyroidism, you will be carefully monitored during your pregnancy and treated with a medication called levothyroxine (Levoxyl, Synthroid, and Unithroid).

Subclinical hypothyroidism is usually asymptomatic, which means that it typically doesn’t cause symptoms.

However, subclinical hypothyroidism symptoms are possible. These symptoms may include:

Experts aren’t sure what causes subclinical hypothyroidism in pregnancy, but several factors may be at play. First of all, thyroid function may be affected by the hormonal changes that happen in pregnancy, such as increased estrogen and human chorionic gonadotropin.

Genetics, older age, a higher body mass index, and a diet low in iodine may also contribute to the development of subclinical hypothyroidism.

Anyone can get subclinical hypothyroidism during pregnancy, but certain individuals may be at increased risk. Risk factors include:

  • living in an area where iodine deficiency is common
  • having a history of thyroid disease
  • having a family history of thyroid disease
  • being positive for thyroid antibodies
  • having type 1 diabetes
  • having an autoimmune disease
  • having a previous miscarriage, premature delivery, or a history of infertility
  • history of using the medications lithium or amiodarone
  • history of two or more previous pregnancies
  • prior radiation exposure of the neck or head
  • BMI over 40
  • being age 30 or older

Overt hypothyroidism during pregnancy can cause serious complications, such as pregnancy loss, premature birth, placental abruption, high blood pressure, and other concerns. However, it’s much less clear if subclinical hypothyroidism causes similar issues.

The evidence showing that subclinical hypothyroidism is connected to pregnancy complications is not consistent, and some studies show a connection while others do not. Complications that have come up in studies include:

In most cases, subclinical hypothyroidism in pregnancy doesn’t require treatment.

Current American Thyroid Association (ATA) guidelines suggest that treatment should occur when an elevated TSH level is coupled with signs of thyroid autoimmune disease, which is detected by the presence of thyroid peroxidase antibodies.

These antibodies may be elevated in people with other thyroid conditions, such as Hashimoto’s thyroiditis or Graves’ disease. It’s estimated that about half of all people with subclinical hypothyroidism during pregnancy have thyroid peroxidase antibodies as well.

When treatment is required, it involves taking a medication called levothyroxine (Levoxyl, Synthroid, and Unithroid), which is a common treatment for thyroid disorders.

Your healthcare professional will give you instructions for how to take this medication, but it’s usually advised that you take the medication at a different time than your prenatal vitamins or iron supplements.

You can usually stop taking levothyroxine after you have your baby. However, your healthcare professional can advise you exactly when to stop the medication.

Most pregnant individuals will not have any long lasting issues as a result of being diagnosed with subclinical hypothyroidism in pregnancy. Again, the studies we have about the outcome of subclinical hypothyroidism are inconsistent.

Possible short- and long-term outcomes of subclinical hypothyroidism include:

  • miscarriage and pregnancy loss
  • premature delivery
  • gestational diabetes
  • high blood pressure
  • cognitive issues in children
  • eclampsia of pregnancy
  • placenta abruption
  • low birth weight

Some studies that looked at these outcomes showed that they were more likely in people whose subclinical hypothyroidism wasn’t treated. But more research needs to be done to determine the accuracy of this information.

Not everyone is screened for subclinical hypothyroidism in pregnancy. However, if you have a known risk factor for it, you will likely be screened. Screening involves a blood test that looks at your thyroid hormone levels.

Subclinical hypothyroidism is diagnosed when you have elevated TSH levels but normal T4 levels. TSH is usually considered to be elevated when it is above 2.5 milliunits per liter.

Subclinical hypothyroidism is a condition affecting up to 8% of people of reproductive age. It can affect pregnant individuals, but the research on the complications it may cause is unclear. The condition is often not treated. It’s usually only treated when people have thyroid peroxidase antibodies and elevated TSH levels.

Reach out to your healthcare professional if you have questions about subclinical hypothyroidism and how it may affect your pregnancy.