Thyroid Disease & Pregnancy

*UPDATED 10/2022

What is the thyroid?

Maybe you learned about the thyroid gland in health or anatomy and physiology class, but haven’t thought much else about it since your school days. This small but mighty part of our bodies plays a big role in our overall health, and there are important considerations for thyroid function during pregnancy. The thyroid gland, found near the lower part of the throat near the trachea, creates and produces thyroid hormones that are needed throughout many systems in the body. Those who have a thyroid producing too much or too little of these hormones have thyroid disease.


When the thyroid gland produces too much hormone, a condition called ‘hyperthyroidism’ is present. When there is too little hormone produced, it is known as ‘hypothyroidism’. Today, I’ll share the recent ACOG recommendations for thyroid disease in pregnancy so you can be better informed when it comes to your thyroid health and your pregnancy.

Hyperthyroidism: Low TSH, High Free T4

A diagnosis of hyperthyroidism with low thyroid stimulating hormone (TSH) and increased free T4 (FT4) is based on clinical symptoms and abnormal laboratory values. Hyperthyroidism occurs in 0.2-0.7% of pregnancies with Graves disease accounting for 95% of these cases. If left untreated during pregnancy, hyperthyroidism can potentially lead to preterm delivery, low birth weight, stillbirth, miscarriage, preeclampsia, maternal heart failure, and a potentially life-threatening condition called ‘thyroid storm’.


The management of a new diagnosis of hyperthyroidism in pregnancy generally follows these principles:

  • First trimester, take Propylthiouracil (PTU): 100 to 600mg divided three times a day
  • After the first trimester, take PTU: 100 to 600mg divided three times a day or Methimazole (MMI) 5 to 30mg divided into twice a day
  • Follow free T4 and total T3 every 2 to 4 weeks until values are titrated to high normal range

It is not ideal to use MMI in the first trimester due to its association with birth defects, including esophageal/choanal atresia and aplasia cutis. Due to the rare association of PTU with hepatotoxicity (toxicity to the liver), one may be given the option to transition to MMI or continue PTU after the first trimester.

A very rare (<1%) side effect with PTU or MMI treatment is agranulocytosis, which involves having an extremely low number of granulocytes in the blood. You should stay alert for sore throat or fever, and contact your healthcare provider immediately if they occur within 3 months of starting treatment of PTU or MMI.

Some patients will experience heart palpitations. If this occurs, a medication called propranolol may be given to help control symptoms.

How Can Hyperthyroidism Affect the Fetus?

Fetal thyrotoxicosis is when the fetus in utero develops a clinical state of high levels of circulating thyroid hormones (T3 and/or T4) in the body from any cause. Because of the persistence of maternal antibodies, the possibility of fetal thyrotoxicosis should be considered in all patients with a history of Graves disease. Signs of fetal thyrotoxicosis are fetal hydrops, fetal growth restriction, and fetal goiter that can be found on ultrasound or fetal tachycardia (high heart rate) that can be seen on ultrasound or in the clinic when assessing fetal heart rate.

Hypothyroidism: High TSH, Low Free T4

The management of a new diagnosis of hypothyroidism in pregnancy is based on clinical symptoms and abnormal laboratory values. It occurs in 2-10 per 1,000 pregnancies.The most common type of hypothyroidism is Hashimoto thyroiditis where antithyroid peroxidase antibodies destroy the thyroid gland. Unlike in Graves disease, maternal antibodies rarely cross the placenta to affect the fetus. If left untreated, however, the potential outcomes for hypothyroidism are miscarriage, preeclampsia, preterm birth, abruption, stillbirth, low birth weight, and abnormal neuropsychological development in offspring.


The management of of a new diagnosis of hypothyroidism in pregnancy generally follows these principles:

  • Follow TSH every 4-6 weeks and titrate to lower reference limit
  • The most common medication prescribed is Levothyroxine: 1 to 2 micrograms/kg daily (typically 100 micrograms daily)
  • It’s recommended to avoid T3 compounds (desiccated thyroid extract or synthetic T3)

How Can Hypothyroidism Affect the Fetus?

Fetal central nervous system development is dependent on adequate maternal thyroid hormones. It is rare for maternal antibodies to cross the placenta and cause fetal hypothyroidism.

Subclinical Hyperthyroidism & Hypothyroidism

Subclinical hyperthyroidism is an abnormally low serum TSH concentration with free T4 levels within the normal ref range. It has not been associated with adverse pregnancy outcomes. Treatment of pregnant persons with subclinical hyperthyroidism is not recommended because there is no benefit to the patient or fetus.

Subclinical hypothyroidism is an elevated serum TSH level in the presence of a normal free T4 level. Again, there is no evidence that identification and treatment of subclinical hypothyroidism during pregnancy improves outcomes so no treatment is recommended.

Thyroid Testing in Pregnancy

The American College of Obstetricians and Gynecologists, the Endocrine Society, and the American Association of Clinical Endocrinologists recommend against universal screening for thyroid disease in pregnancy. They recommend testing during pregnancy only for patients who are at increased risk of overt hypothyroidism. Indicated testing of thyroid function should be performed in patients with a personal or family history of thyroid disease, type 1 diabetes mellitus, or clinical suspicion of thyroid disease. Speak with your physician if you feel like you are at risk for thyroid disease, have known thyroid disease, or have a family history of it. If you are not already working with an endocrinologist, your OBGYN might refer you to one.

Summary of Recommendations from ACOG PB 223: "Thyroid Disease in Pregnancy":

The following recommendations are based on good and consistent scientific evidence (Level A):

  • Universal screening for thyroid disease in pregnancy is not recommended because identification and treatment of maternal subclinical hypothyroidism has not been shown to result in improved pregnancy outcomes and neurocognitive function in offspring.
  • If indicated, the first-line screening test to assess thyroid status should be measurement of the TSH level.
  • The TSH level should be monitored in pregnant women being treated for hypothyroidism, and the dose of levothyroxine should be adjusted accordingly with a goal TSH level between the lower limit of the reference range and 2.5 milliunits/L. Thyroid-stimulating hormone typically is evaluated every 4–6 weeks while adjusting medications.
  • Pregnant women with overt hypothyroidism should be treated with adequate thyroid hormone replacement to minimize the risk of adverse outcomes.
  • The level of fre should be monitored in pregnant women being treated for hyperthyroidism, and the dose of antithyroid drug (thioamide) should be adjusted accordingly to achieve a free T at the upper end of the normal pregnancy range. Among women who also have thyrotoxicosis, total T should be monitored with a goal level at the upper end of normal pregnancy range.
  • Pregnant women with overt hyperthyroidism should be treated with antithyroid drugs (thioamides).

The following recommendation is based on limited or inconsistent scientific evidence (Level B):

  • Either propylthiouracil or methimazole, both thioamides, can be used to treat pregnant women with overt hyperthyroidism. The choice of medication is dependent on trimester of pregnancy, response to prior therapy, and whether the thyrotoxicosis is predominantly T or T

The following recommendations are based primarily on consensus and expert opinion (Level C):

  • Indicated testing of thyroid function should be performed in women with a personal or family history of thyroid disease, type 1 diabetes mellitus, or clinical suspicion of thyroid disease.
  • Measurements of thyroid function are not recommended in patients with hyperemesis gravidarum unless other signs of overt hyperthyroidism are evident.


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