Addressing Thyroid Disorders During Pregnancy: Diagnosis and Treatments

Thyroid disease is a widespread condition in women of childbearing age, second only to diabetes.[4] Control of thyroid disease is important before and during pregnancy. Uncontrolled thyroid disease can decrease fertility as well as increase the risk of miscarriage. It can also increase the risk of low birth weight and preterm birth. [4,19] This article will discuss thyroid disease symptoms, diagnosis, and management during pregnancy.  

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The Importance of Thyroid Function in Pregnancy

Proper thyroid hormone balance is essential for the health of both the mother and the developing baby. The fetal thyroid doesn't start to produce adequate thyroid hormones until 20 weeks of gestation, and the maternal thyroid gland provides for both until this time.[8] This requires an increase in thyroid hormone production during pregnancy. During pregnancy, close monitoring of thyroid hormone levels is important for women with thyroid disorders.  

Untreated or sub-optimally treated thyroid disease can have significant impacts on maternal and fetal health. Some of the conditions associated with hypothyroidism in pregnancy include[19]:

• Elevated blood pressure leading to pre-eclampsia
• Anemia
• Miscarriage
• Low birth weight
• Stillbirth
• Congestive heart failure
• Low IQ or problems with neurological development

Hyperthyroidism is more likely to lead to congestive heart failure in pregnant women, up to 10% if left untreated.[16] Other potential complications of hyperthyroidism in pregnancy include[16]:

• High blood pressure and pre-eclampsia
• Placental abruption
• Preterm labor
• Thyroid storm
• Low birth weight
• Goiter
• Rapid heart rate
• Fetal hydrops
• Fetal cardiac failure
• Early bone maturation
• Intrauterine growth restriction
• Neurodevelopmental abnormalities

Due to the potentially severe consequences of thyroid disorders in pregnancy, it's especially important to manage thyroid disorders in all stages of pregnancy optimally.   

Common Thyroid Disorders in Pregnancy

Hypothyroidism is the most common type of thyroid disease seen in pregnancy. The incidence of hypothyroidism during pregnancy is 0.3% to 0.5% for clinically significant hypothyroidism and 2 to 3% for subclinical cases.[4] In parts of the world where nutritional deficiencies are more common, iodine deficiency is the most common cause of hypothyroidism. In parts of the world where iodine is not a concern, Hashimoto thyroiditis is the most common type of hypothyroidism to present in pregnancy.[15] Hashimoto thyroiditis is an autoimmune form of thyroid disorder.  

Symptoms of hypothyroidism include[6]:

• Fatigue
• Weight gain
• Intolerance to cold
• Dry skin and brittle nails
• Hair loss
• Constipation
• Muscle aches and joint pain
• Irregular and/or heavy menstrual periods
• Brain fog
• Mood problems
• Memory problems

Hyperthyroidism in pregnancy is usually due to Graves’ disease, though there are a few other types of hyperthyroidism that may be seen. The overall prevalence of hyperthyroidism in pregnancy is 0.1% to 0.2%.[14] Other conditions that cause hyperthyroidism include gestational transient thyrotoxicosis, hydatidiform molar pregnancy, thyroid toxic adenoma or multinodular goiter, DeQuervain subacute thyroiditis, thyroid hormone receptor mutations, some forms of tumors, and overtreatment of thyroid medications to treat hypothyroidism.[16]

Symptoms of hyperthyroidism include[17]:

• Fast heartbeat
• Irregular heartbeat
• Palpitations
• Excessive sweating
• Feeling irritable, anxious, or nervous
• Increased appetite
• Fatigue
• Weakness
• Enlargement or tenderness of the thyroid gland
• Thin skin
• Brittle hair and nails
• Menstrual problems
• Infertility
• Increased frequency of bowel movements

The third type of thyroid disorder associated with pregnancy is postpartum thyroiditis. This thyroid disorder occurs within the first year after delivery in women who haven’t had thyroid disease previously and may be temporary or permanent.[13] Postpartum thyroiditis develops during pregnancy, with thyroid peroxidase antibodies forming during pregnancy in many women. The incidence of postpartum thyroiditis is around 5%.[13] Postpartum thyroiditis can have a hyperthyroid phase and a hypothyroid phase.

The symptoms of postpartum thyroiditis include irritability, palpitations, fatigue, and heat intolerance in the hyperthyroid stage. The hypothyroid stage symptoms include constipation, dry skin, fatigue, concentration problems, cold intolerance, and numbness or tingling.[13]

Diagnosing Thyroid Disorders During Pregnancy

The American College of Obstetrics and Gynecology (ACOG) does not recommend universal screening in pregnancy primarily because research has not shown improved pregnancy outcomes or neurocognitive function when treating subclinical hypothyroidism.[20] They do recommend testing any woman with a personal or family history of thyroid disease, type 1 diabetes mellitus, or anyone for whom there is a suspicion of thyroid disease. This includes women with a goiter or thyroid nodules but doesn’t include women with a mildly enlarged thyroid gland because the thyroid gland normally enlarges with pregnancy.[20] For women with known thyroid disease, ACOG recommends monitoring with a thyroid stimulating hormone (TSH) level every 4-6 weeks.

The American Thyroid Association recommends testing every four weeks or with any dose changes in the first half of pregnancy. The frequency of testing may decrease in the second half of pregnancy as long as the levels are stable and within the target range.[3] In addition, they recommend checking thyroid receptor binding antibody (TRBA) levels in women treated with levothyroxine following previous treatment for Graves’ disease. If these antibodies are positive, close follow-up with fetal ultrasounds and postnatal thyroid function testing in the baby is necessary.[3]

The first line screening test for thyroid disease is a TSH level. This is also the test used for monitoring the treatment of hypothyroidism. Free T₄ levels should be monitored in women who are being treated for hyperthyroidism and to distinguish between overt and subclinical hypothyroidism. Total T₃ levels should be monitored in women who have T₃ toxicosis.[20]  Normal thyroid hormone levels during pregnancy based on the trimester. [4]

Thyroid hormone antibodies may also be indicated if there is a concern for autoimmune thyroid disease. Thyroid peroxidase antibodies (TPOAb) are elevated in people with Hashimoto’s thyroiditis and some patients with Graves’ disease. Thyroglobulin antibodies (Tg Ab) are sometimes elevated in Hashimoto’s thyroiditis but are more often used to monitor patients treated for thyroid cancer. Thyroid-stimulating hormone receptor antibodies (TSHRAb or TRAb) are elevated in people with Graves’ disease. Thyroid Stimulating Immunoglobulin (TSI) is also raised in Graves’ disease.[3] 

Thyroid testing can be challenging in pregnancy due to the physiologic changes. Both human chorionic gonadotropin (hCG) and estrogen stimulate the thyroid, increasing thyroid hormone levels in the blood and enlarging the thyroid gland.[19] Many of the symptoms of thyroid disease are similar to those of pregnancy, such as fatigue and weight gain.  

Treatment Strategies for Thyroid Disorders in Pregnancy

Hypothyroidism in pregnancy is generally treated with levothyroxine. The American Thyroid Association recommends against continuing or starting desiccated thyroid or triiodothyronine (T₃) medications during pregnancy. This is because T₃ cannot cross the placenta, and using these medications may decrease the amount of maternal T₄ that crosses the placenta. T₄ is vital for the development of the fetal brain.[3]

The American Thyroid Association and American Association of Clinical Endocrinologists recommend starting T₄ replacement therapy with doses of 1-2 micrograms/kg daily.[20] Dose adjustments are based on the TSH levels. The goal is to keep the TSH level between the lower level for the stage of pregnancy and 2.5milliuits/L.  

Hyperthyroidism in pregnancy is treated with a class of medications called thioamides.[20] The choice of medication varies based on the trimester of pregnancy and how the patient has previously responded to medications. It may also depend on whether the hyperthyroidism is related to T₄ or T₃. Methimazole is avoided in the first trimester because of risks for fetal malformations. Beyond the first trimester, methimazole or propylthiouracil is used.  

Propylthiouracil decreases the conversion of T₄ to T₃  and can have some liver toxicity,_, which may make it a better choice for people with T₃ toxicosis.[10] The goal for treating hyperthyroidism is maintaining the T₄ levels at the high-normal to slightly above goal range. For women with T₃ toxicosis, the total T₃ level should be monitored.  In addition to treating thyroid hormone levels, beta-blockers such as propranolol can be used to control symptoms such as palpitations.[20]

For women with thyroid disorders, regular monitoring is important during pregnancy. As previously mentioned, prior to 20 weeks, monitoring is recommended every 4-6 weeks[20,3]. In the second half of pregnancy, monitoring frequency may depend on how stable and well-controlled the levels have been early in pregnancy.  

Several herbal and dietary supplements are sometimes used by people with thyroid disease. The American Thyroid Association notes that complementary and alternative medicine (CAM) therapies should be considered with caution and discussed with your healthcare team. There is limited evidence on the safety and efficacy of these treatments.[2]  While there is limited data on CAM therapies in general, this is even more true in pregnancy. 

Multidisciplinary Care Approach

In the nonpregnant patient, thyroid disease may be managed by a primary care provider or endocrinologist. During pregnancy, management can be a little more challenging because there are two patients to care for and different physiologic dynamics. In pregnant women with thyroid disease, the management team may include an obstetrician and an endocrinologist, especially in the case of hyperthyroidism.

A high-risk pregnancy specialist may be involved if more frequent monitoring of the fetus is necessary. It is also important that the pediatrician be notified at the time of delivery as the baby may need additional monitoring.

Patient Education and Support

As with any chronic condition, it is important to optimize management before pregnancy whenever possible. Patients with thyroid disorders should receive education on the importance of medication compliance, as tight control of thyroid hormone levels is essential for the health of the mother and baby.

Patients should be educated on the symptoms of both hyperthyroidism and hypothyroidism, which would indicate a need to check levels. Treatment needs may also change in the postpartum period, so patients are advised to continue monitoring their symptoms even after delivery.  

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Key Takeaways

• Thyroid disorders are the second most common endocrine disorders in pregnancy.
• Optimizing treatment prior to pregnancy is recommended when possible.
• ACOG does not recommend universal screening of pregnant women.
• Thyroid hormone levels should be monitored closely in women who require treatment.
• A multidisciplinary care team is important for the optimal management of both mother and baby.
• Patient education on the importance of treatment adherence and symptom monitoring is essential for all women with thyroid disorders during pregnancy.