*Updated 9/2024
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 or Total T3
A diagnosis of hyperthyroidism is made with clinical symptoms and laboratory testing showing a low yhyroid stimulating hormone (TSH) is low and high free T4 (FT4). Hyperthyroidism occurs in 0.2-0.7% of pregnancies, and Graves disease accounts for 95% of these cases.
How will my hyperthyroidism be treated?
The management of a new diagnosis of hyperthyroidism in pregnancy generally follows these principles:
- In the first trimester, take propylthiouracil (PTU): 100 to 600mg divided dose 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 (if T3 thyrotoxicosis is present) every 2 to 4 weeks until values are titrated to a free T4 or total T3 in the 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. Signs of hepatotoxicity include dark urine, pale stools, and jaundice. Your provider will get baseline liver function tests when starting PTU.
A very rare (<1%) side effect with PTU or MMI treatment is agranulocytosis, which involves having an extremely low number of granulocytes (white blood cells or infection-fighting cells) in the blood. You should stay alert for sore throat or fever, and contact your healthcare provider immediately if this occurs within 3 months of starting treatment of PTU or MMI. If you have these symptoms, the PTU or MMI will be stopped. Your provider will get baseline labs before starting PTU or MMI.
Transient leukopenia (a lower number of white blood cells) occurs in up to 10% of pregnant patients who take PTU or MMI, but this situation does not require stopping the medication.
Some patients will experience heart palpitations with uncontrolled hyperthyroidism. If this occurs, a medication called propranolol may be given to help control symptoms.
How can uncontrolled, untreated or undertreated hyperthyroidism affect the pregnant individual?
Inadequately treated maternal thyrotoxicosis associated with an increased risk of:
- Preeclampsia with severe features
- Maternal heart failure
- Thyroid storm
How can uncontrolled, untreated or undertreated hyperthyroidism affect the fetus and neonate?
Inadequate treatment is associated with:
- Medically-indicated preterm delivery
- Low birth weight
- Pregnancy loss
- Stillbirth
Fetal thyrotoxicosis:
Fetal thyrotoxicosis is when the fetus develops a clinical state of high levels of circulating thyroid hormones (T3 and/or T4). Because of the persistence of maternal antibodies associated with thyroid disease, 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.
Neonatal complications include:
- Irritability
- Failure to thrive
- Hyperkinesis
- Diarrhea
- Poor feeding
- Jaundice
- Thrombocytopenia-low platelets
- Advanced bone age
- Craniosynostosis
- Remission by 20 weeks is common
- Often resolved by 48 weeks
Will my pregnancy care change because of my diagnosis of hyperthyroidism?
Antenatal testing with nonstress tests and/or biophysical profiles can be considered with poorly controlled disease. Ultrasounds assessing fetal growth can be considered starting at 32 weeks. There is no recommendation for routine induction of labor cesarean delivery for well-controlled disease.
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 laboratory diagnosis is made with a high TSH and low free T4. A goiter may or may not be present. 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.
How will my hypothyroidism be treated?
The management of of a new diagnosis of hypothyroidism in pregnancy generally follows these principles:
- Follow TSH every 4-6 weeks and titrate to a TSH level in the lower reference range 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 uncontrolled, untreated or undertreated hypothyroidism affect the pregnant individual?
Inadequate treatment is associated with:
- Preeclampsia
- Placental abruption
- Pospartum hemorrhage
How can uncontrolled, untreated or undertreated hypothyroidism affect the fetus and neonate?
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. If hypothyroidism is left untreated, however, the potential outcomes include:
- Pregnancy loss
- Preterm birth
- Stillbirth
- Low birth weight
- Abnormal neuropsychological development in offspring
- The risk of each of these complications is 2-3 fold higher compared to pregnant patients with controlled hypothyroidism.
Will my pregnancy care change because of my diagnosis of hypothyroidism?
The same considerations as stated for hyperthyroidism can be made here.
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.
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 patients 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 patients with overt hypothyroidism should be treated with adequate thyroid hormone replacement to minimize the risk of adverse outcomes.
- The level of free T4 should be monitored in pregnant patients being treated for hyperthyroidism, and the dose of antithyroid drug (thioamide) should be adjusted accordingly to achieve a free T4 at the upper end of the 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 patients 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.
Resources:
- The OBG Project- ACOG Update: Thyroid Disease in Pregnancy
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