What is Hyperthyroidism in Pregnancy?
Hyperthyroidism, while not very common, can affect between 0.1% to 0.4% of pregnancies. This condition is characterized by too much thyroid hormone, specifically T4 and T3, in the body and too little of the hormone called thyroid-stimulating hormone (TSH) in the blood. Although not very common, it’s critical to identify and treat serious hyperthyroidism as soon as possible to reduce the risk of complications for both the pregnant person and the unborn baby.
Ideally, we would like to diagnose hyperthyroidism before someone gets pregnant and start treatment to normalize the thyroid hormone levels in the body. However, since about half of all pregnancies in the United States aren’t planned, it’s crucial to diagnose thyroid problems early.
In this discussion, we’ll talk about the causes, frequency, how the body responds to it, and how we first evaluate hyperthyroidism during pregnancy. Following this, we’ll discuss the treatment options, how to manage the condition, and the possible complications associated with it.
What Causes Hyperthyroidism in Pregnancy?
Hyperthyroidism, a condition where the thyroid gland produces too much thyroid hormone, can occur during pregnancy and requires treatment. In most cases, this is due to a condition known as Grave’s disease, which is responsible for about 85% to 95% of significant hyperthyroidism cases. Grave’s disease is an autoimmune condition, which means the body’s immune system mistakenly attacks the body’s own cells.
In this case, the immune system produces certain antibodies, known as thyrotropin (TSH)-receptor antibodies (TRAb). These antibodies can bind to receptors in the thyroid gland, which then causes the gland to produce more thyroid hormones than necessary.
Risk Factors and Frequency for Hyperthyroidism in Pregnancy
Grave’s disease is a condition that affects about 0.5% of the population. This disease often develops in women who are between the ages of 20 and 40, and the chances of getting it increase with age. Hyperthyroidism (an overactive thyroid) during pregnancy is relatively rare, affecting about 0.1% to 0.2% of pregnancies.
There are other causes of hyperthyroidism, but they are generally less common. For instance, it’s quite rare for women under 40 to develop this condition due to thyroid nodules, with an occurrence rate of only about 0.001% to 0.002%. However, in areas where iodine deficiency is common, the rate of hyperthyroidism may be higher. This is because a lack of iodine can lead to the development of functional thyroid nodules (lumps in the thyroid gland).
Signs and Symptoms of Hyperthyroidism in Pregnancy
Hyperthyroidism is a medical condition and some of its symptoms are similar to normal body changes during pregnancy, such as an increased heart rate and shortness of breath. Other signs of hyperthyroidism include:
- Sweating excessively
- Intolerance to heat
- Heart palpitations
- Inability to sleep (insomnia)
- Frequent bowel movements
- Feeling nervous
- Increased hunger
- Itchy skin (pruritus)
- Anxiety
- Trembling hands
During a physical exam, a doctor may find an enlarged thyroid gland (goiter) and high blood pressure. If you have Graves’ disease, a type of hyperthyroidism, you might experience bulging eyes (exophthalmos or proptosis) about 50% of the time, and a specific kind of swelling in the leg (pretibial myxedema) less than 10% of the time.
It’s crucial to know if you have previously been diagnosed with Graves’ disease, even if you’ve had surgery or treatment to eliminate thyroid function (radioiodine ablation). This is because certain antibodies that can affect the thyroid may still be present and cause hyperthyroidism in your baby during fetal development.
Testing for Hyperthyroidism in Pregnancy
If you are pregnant and your doctor suspects you might have an overactive thyroid (hyperthyroidism), they will likely perform some laboratory tests. These are generally the same tests as those given to non-pregnant patients.
Your doctor will start by checking your TSH (thyroid-stimulating hormone) levels. TSH levels are often lower in pregnant women, as an increase in a hormone called HCG stimulates the thyroid, leading to a drop in TSH. After this, your doctor will measure your thyroid hormone levels to determine if you have overt (obvious) hyperthyroidism, or whether the changes are a normal part of pregnancy.
However, it’s important to note that standard tests might not always provide a clear diagnosis because hormone levels naturally vary during pregnancy. Due to this, your doctor might decide to monitor you closely, repeatedly checking your thyroid levels, rather than starting you immediately on medication.
In most cases, it may not be harmful to have mild hyperthyroidism during pregnancy. However, to confirm a diagnosis, your doctor could measure your levels of thyroid receptor antibodies (TRAbs), a type of protein usually present in a condition known as Grave’s disease. Bear in mind that standard tests cannot tell the difference between stimulating and inhibiting TRAbs, so if there’s any confusion about your condition, your doctor might order a more sensitive test.
It’s also recommended to screen for TRAbs in any pregnant woman who had Graves’ disease in the past, has had a baby affected by Graves’ disease, or has gone through a recent radioiodine treatment or thyroid surgery. This usually occurs between weeks 20 to 24 of your pregnancy.
Treatment Options for Hyperthyroidism in Pregnancy
Hyperthyroidism during pregnancy is typically treated with medications that reduce the excessive production of thyroid hormones. The most common drugs used in the U.S. for this purpose are called antithyroid drugs (ATD), specifically thioamides, propylthiouracil (PTU), and methimazole (MMI). Another drug named carbimazole, which acts similarly to methimazole, is often used outside of North America. These drugs are capable of passing through the placenta and affecting the unborn child.
Previously, PTU was widely used for treating hyperthyroidism in all patients. However, PTU can potentially cause liver damage that could lead to liver failure, necessitating a liver transplant. As a result, methimazole is now more widely used if patients can tolerate it. However, during early pregnancy, methimazole and carbimazole are usually avoided due to the risk of rare birth defects. After the first trimester, patients are usually switched to methimazole to lower the risk of liver damage.
Hyperthyroidism can pose a threat to the mother’s health and increase the risk of losing the baby if left untreated. So, even though antithyroid drugs carry potential risks, treatment is typically still necessary. If a patient can’t tolerate PTU, methimazole is preferred as a treatment option, even during the first trimester.
When initiating treatment, adjusting doses, or switching between drugs, regular thyroid function tests should be obtained every two to four weeks to ensure that the thyroid hormone levels remain balanced.
There are other ways of treating hyperthyroidism as well, including potassium iodide and surgery. These options are typically only considered if antithyroid drugs aren’t effective or aren’t tolerated by the patient. Surgery is usually only an option for patients who experience negative side effects due to the swelling of their thyroid, known as a goiter. If surgery is necessary, it’s ideally performed in the second trimester when the risk to the baby is lowest.
Beta-blockers like propranolol can also be used temporarily to help control symptoms until a balanced thyroid state is achieved. But these drugs should be discontinued once thyroid hormone levels are stable due to the risk of developing other complications.
Regarding the baby’s health, it’s important for doctors to carefully monitor the baby throughout the pregnancy. Ultrasound scans can help screen for any signs of thyroid dysfunction in the baby, which might be indicated by an enlarged thyroid or delayed growth. Regular thyroid function tests should be done during pregnancy to monitor the risk of hyperthyroidism in the unborn baby. Additional monitoring may be necessary in some cases.
What else can Hyperthyroidism in Pregnancy be?
Grave disease is the main cause of significantly high thyroid levels in pregnant women, although there are other causes. These other causes play a vital role in determining whether treatment is necessary.
The most common cause of temporary high thyroid levels during pregnancy is gestational transient thyrotoxicosis (GTT), also referred to as transient gestational hyperthyroidism (TGH). This condition impacts 1% to 3% of pregnancies, occurring more frequently than Grave disease during pregnancy. This temporary increase in thyroid levels typically resolves by itself between the 14th to 20th weeks of pregnancy, and doesn’t require treatment.
Interestingly, GTT is often linked with severe nausea and vomiting that can be as intense as hyperemesis gravidarum. 50% to 70% of women suffering from hyperemesis gravidarum also exhibit high thyroid levels. GTT can be differentiated from Grave-induced high thyroid levels by a lack of TRAbs. GTT doesn’t cause swelling in the thyroid gland or eyes, and the texture of the thyroid appears normal when examined with ultrasound.
Another cause of high thyroid levels is Hydatidiform molar pregnancies, which are a type of gestational trophoblastic disease. The only treatment is complete removal of the molar pregnancy.
Other causes include single toxic adenoma and toxic multinodular goiter, which involve autonomous nodules that produce thyroid hormones. These nodules usually appear in women over 40 years old and produce less thyroid hormones than Grave disease. This is why anti-thyroid drugs may not be necessary.
Subacute thyroiditis, also known as DeQuervain subacute thyroiditis, is a rare cause of thyroid inflammation induced by a viral infection. It can cause an increase in thyroid hormones. Likewise, mutations in the thyroid hormone receptor can increase thyroid hormone circulation and exposure to the fetus, heightening the risk of spontaneous abortion. Hyperthyroidism can also result from an excessive intake of levothyroxine, a medication used to treat hypothyroidism.
Lastly, some rare neoplastic causes of high thyroid levels in pregnancy include Struma ovarii, a type of ovarian teratoma that contains functional thyroid tissue, a TSH-producing pituitary adenoma, which is a benign tumor capable of producing thyroid hormones, and metastatic lesions from thyroid cancer that can produce thyroid hormones.
What to expect with Hyperthyroidism in Pregnancy
With proper treatment and regular health checks, pregnant women with hyperthyroidism can expect better pregnancy outcomes and fewer health problems. Women with poorly managed hyperthyroidism are at the greatest risk of complications, with about 10% facing the possibility of heart failure if left untreated. Further, the life-threatening condition known as a thyroid storm can occur in 1% to 2% of pregnant women with hyperthyroidism.
Interestingly, many pregnant women with a type of hyperthyroidism called Grave’s disease often see a remission or reduction in their symptoms towards the end of the pregnancy. This happens because pregnancy naturally suppresses the immune system and results in a decrease in TRAb (thyroid-stimulating antibodies) levels. Sometimes, these antibodies might even switch from increasing thyroid activity to reducing it, thus easing the disease symptoms.
On the other hand, there is an increased risk of hyperthyroidism symptoms returning or getting worse in the 3 to 18 months following childbirth. This is because the immune system gets back to its usual activity, with the risk being highest about seven to nine months after giving birth. Most women who had their Grave’s disease under control before pregnancy may see the disease return or experience thyroid inflammation after childbirth.
Possible Complications When Diagnosed with Hyperthyroidism in Pregnancy
Treating overactive thyroid during pregnancy is crucial to reduce the potential for complications in both the mother and baby. Complications for the mother can include increased risk of miscarriage, high blood pressure during pregnancy, preeclampsia, premature detachment of the placenta, and premature labor. Problems can escalate if the mother’s thyroid becomes excessively overactive leading to a thyroid storm, which can result in heart failure, the need for intensive care, and even death.
For the developing baby, balanced thyroid hormones are crucial as they control the development of the brain and the growth of nerve cells. If the mother has an overactive thyroid, this can lead to complications in the baby such as premature birth, low birth weight, enlarged thyroid, rapid heartbeat, accumulation of fluid in body tissues, heart failure, early bone maturation, restricted growth in the womb, and abnormalities in brain development. These effects can occur due to an excess of thyroid hormone passing from the mother to the baby via the placenta, or if Antibodies that Stimulate the Thyroid (TRAbs) activate the baby’s thyroid. The risk of these conditions increases as the concentration of TRAbs in the mother’s body increases.
Overuse of drugs to treat overactive thyroid during pregnancy can lead to an underactive thyroid in the baby. However, if treatment is adequate, a mother with an underactive thyroid will usually give birth to a baby with a normal thyroid. But, if the mother had Antibodies that Stimulate the Thyroid (TRAbs) during pregnancy, they will still be in the baby’s system after birth. It takes two to three days for the baby to metabolize the thyroid drugs, during which period, the TRAbs may trigger an overactive thyroid in the baby. This happens in 1.5 to 2% of babies born to mothers with Grave’s disease and can last for a few weeks to four to six months. If it persists, it can cause heart failure, liver problems, small head size, premature closing of the gaps between the bones in the baby’s skull, high blood pressure in the lungs, abnormal blood clotting, and intellectual disability.
A thyroid storm, a dangerous condition caused by an excessively overactive thyroid, can be life-threatening. This condition usually arises due to a triggering event such as labor, cesarean section, preeclampsia, trauma, or infection. Symptoms may include a rapid heart rate, irregular heartbeat, changes in mental status, intolerance to heat, fever, nausea and vomiting, and heart failure. The treatment requires comprehensive care and includes medications to reduce the production of thyroid hormones, replenish electrolytes, rehydrate, and cool down the body. If heart failure is present, medication can be given to increase heart function, and it’s also crucial to treat the event that triggered the thyroid storm.
Postpartum thyroiditis is a condition that can occur within six weeks and up to a year after childbirth due to a rebound of the immune system following the natural immune suppression of pregnancy. This condition can cause temporary overactivity of the thyroid due to damage to thyroid tissue leading to a surge in thyroid hormones. This is usually followed by a period of underactivity of the thyroid, which may persist.
Medical Complications:
- Maternal Death
- Heart Failure
- Hyper or Hypothyroidism (in both mother and baby)
- Mental Status Changes
- High Blood Pressure in Lungs
- Abnormal Blood Clotting
- Intellectual Disability in Baby
Preventing Hyperthyroidism in Pregnancy
Women planning to become pregnant often ask for advice before conceiving. For those with a condition called Grave’s disease, it’s crucial to discuss treatment options beforehand. Grave’s disease can cause an overly active thyroid, or hyperthyroidism, which can impact pregnancy.
Some women might choose to have surgery or a treatment called radioiodine ablation (RAI) to cure their hyperthyroidism. This decision might be especially relevant for women who have high levels of TRAb (an antibody linked with hyperthyroidism) in their blood or a history of thyroid issues in a previous pregnancy. Surgery might be chosen over RAI, as RAI can temporarily increase TRAb levels.
Having surgery or RAI treatment can potentially eliminate the need for certain medications, and avoid their side effects both to the mother and baby, but would require the woman to take thyroid replacement hormones for the rest of her life. These thyroid hormone levels need to be monitored and adjusted with a medication called levothyroxine before pregnancy. However, even after treatment, in some cases, TRAb can cross the placenta and affect the baby’s thyroid; this requires a carefully managed treatment approach.
In general, non-pregnant women with Grave’s disease tend to use a medication called methimazole, as it has a lower risk of liver toxicity than another medication called PTU. Some women might decide to switch from methimazole to PTU before becoming pregnant, due to the associated risks. Women who continue taking methimazole after falling pregnant need to be aware that it can lead to complications if not changed to PTU early in pregnancy. For the baby, these complications from methimazole have been found to be more common than the risk of liver toxicity from PTU.
