Thyroid Disease & Pregnancy

Thyroid disease is a group of disorders that affect the thyroid gland. The butterfly-shaped thyroid gland is located at the front of the neck. It makes hormones responsible for metabolism and brain function, as well as a number of other bodily functions, even the way your heart beats.


Sometimes the thyroid makes too much or too little of these hormones. Too much thyroid hormone is called hyperthyroidism and can cause many of your body’s functions to speed up. “Hyper” means the thyroid is overactive. Too little thyroid hormone is called hypothyroidism and can cause many of your body’s functions to slow down. “Hypo” means the thyroid is underactive.


What role do thyroid hormones play in pregnancy?

Thyroid hormones are crucial for the normal development of your baby’s brain and nervous system. During the first trimester, the first 3 months of pregnancy, your baby depends on your supply of thyroid hormone, which comes through the placenta. At around 12 weeks, your baby’s thyroid starts to work on its own, but it doesn’t make enough thyroid hormone until 18 to 20 weeks of pregnancy.

Two pregnancy-related hormones, human chorionic gonadotropin (hCG) and estrogen, cause higher measured thyroid hormone levels in your blood. The thyroid enlarges slightly in healthy women during pregnancy, but usually not enough for a healthcare professional to feel during a physical exam.

Thyroid problems can be hard to diagnose in pregnancy due to higher levels of thyroid hormones and other symptoms that occur in both pregnancy and thyroid disorders. Some symptoms of hyperthyroidism or hypothyroidism are easier to spot and may prompt your doctor to test you for these thyroid diseases.


Hyperthyroidism in Pregnancy

What are the symptoms of hyperthyroidism in pregnancy?

Some signs and symptoms of hyperthyroidism often occur in normal pregnancies, including faster heart rate, trouble dealing with heat, and tiredness.

Other signs and symptoms can suggest hyperthyroidism:

  • Fast and irregular heartbeat
  • Shaky hands
  • Unexplained weight loss or failure to have normal pregnancy weight gain
  • Feeling too hot
  • Increased sweating
  • Trembling hands
  • Tiredness/fatigue
  • Irritability and anxiety
  • Eye problems, such as irritation or discomfort
  • Menstrual irregularities
  • Infertility


What causes hyperthyroidism during pregnancy?

Hyperthyroidism in pregnancy is usually caused by Graves’ disease and it’s an autoimmune disorder. With this disease, your immune system makes antibodies that cause the thyroid to make too much thyroid hormone. This antibody is called thyroid stimulating immunoglobulin, or TSI.

Graves’ disease may first appear during pregnancy. However, if you already have Graves’ disease, your symptoms could improve in your second and third trimesters. Some parts of your immune system are less active later in pregnancy so your immune system makes less TSI. This may be why the symptoms improve. Graves’ disease often gets worse again in the first few months after your baby is born, when TSI levels go up again. If you have Graves’ disease, your doctor will most likely test your thyroid function monthly throughout your pregnancy and may need to treat your hyperthyroidism. Thyroid hormone levels that are too high can harm your health and your baby’s.

How can hyperthyroidism affect me and my baby?

Untreated hyperthyroidism during pregnancy can lead to

  • miscarriage
  • premature birth
  • low birthweight
  • preeclampsia—a dangerous rise in blood pressure in late pregnancy
  • thyroid storm—a sudden, severe worsening of symptoms
  • Congestive heart failure

Rarely, Graves’ disease may also affect a baby’s thyroid, causing it to make too much thyroid hormone. Even if your hyperthyroidism was cured by radioactive iodine treatment to destroy thyroid cells or surgery to remove your thyroid, your body still makes the TSI antibody. When levels of this antibody are high, TSI may travel to your baby’s bloodstream. Just as TSI caused your own thyroid to make too much thyroid hormone, it can also cause your baby’s thyroid to make too much.

An overactive thyroid in a newborn can lead to

  • a fast heart rate, which can lead to heart failure
  • early closing of the soft spot in the baby’s skull
  • poor weight gain
  • Irritability


How is hyperthyroidism diagnosed?

Your doctor will perform a physical examination and order blood tests to measure your hormone levels. You have hyperthyroidism when the levels of T4 and T3 are higher than normal and the level of TSH is lower than normal. To determine the type of hyperthyroidism you have, your doctor may do a radioactive iodine uptake test to measure how much iodine your thyroid collects from the bloodstream. The thyroid uses iodine to make T3 and T4. Your doctor may also take a picture of your thyroid (a thyroid scan) to see its shape and size and to see whether there is any nodules present.


How do doctors treat hyperthyroidism during pregnancy?

If you have mild hyperthyroidism during pregnancy, you probably won’t need treatment. If your hyperthyroidism is more severe, your doctor may prescribe antithyroid medicines, which cause your thyroid to make less thyroid hormone. This treatment prevents too much of your thyroid hormone from getting into your baby’s bloodstream. Doctors most often treat pregnant women with the antithyroid medicine propylthiouracil  (PTU) during the first 3 months of pregnancy. Another type of antithyroid medicine, methimazole, is easier to take and has fewer side effects, but is slightly more likely to cause serious birth defects than PTU. Birth defects in either type of medicine are rare. Sometimes doctors switch to methimazole after the first trimester of pregnancy. Some women no longer need antithyroid medicine in the third trimester.


Hypothyroidism in Pregnancy

What are the symptoms of hypothyroidism in pregnancy?

Symptoms of an underactive thyroid are often the same for pregnant women as for other people with hypothyroidism. Symptoms include

  • extreme tiredness
  • trouble dealing with cold
  • muscle cramps
  • Severe constipation
  • problems with memory or concentration
  • Weight gain (only 5–10 pounds or 2–4 kg)
  • Dry skin and hair
  • Constipation
  • Menstrual irregularities

Most cases of hypothyroidism in pregnancy are mild and may not have symptoms.

What causes hypothyroidism during pregnancy?

Hypothyroidism in pregnancy is usually caused by Hashimoto’s disease and occurs in 2 to 3 out of every 100 pregnancies.1 Hashimoto’s disease is an autoimmune disorder. In Hashimoto’s disease, the immune system makes antibodies that attack the thyroid, causing inflammation and damage that make it less able to make thyroid hormones.

How can hypothyroidism affect me and my baby?

Untreated hypothyroidism during pregnancy can lead to

  • preeclampsia—a dangerous rise in blood pressure in late pregnancy
  • Anemia
  • miscarriage
  • low birthweight
  • stillbirth
  • Congestive heart failure, rarely

In adults, untreated hypothyroidism leads to poor mental and physical performance. It also can cause high blood cholesterol levels that can lead to heart disease. A life-threatening condition called myxedema coma can develop if severe hypothyroidism is left untreated.

Diagnosis of hypothyroidism is especially important in pregnancy. Untreated hypothyroidism in the mother may affect the baby’s growth and brain development.

All babies are tested at birth for hypothyroidism. If not treated promptly, a child with hypothyroidism could have an intellectual disability or fail to grow normally.


 How do doctors treat hypothyroidism during pregnancy?

Treatment for hypothyroidism involves replacing the hormone that your own thyroid can no longer make. Your doctor will most likely prescribe levothyroxine, a thyroid hormone medicine that is the same as T4, one of the hormones the thyroid normally makes. Levothyroxine is safe for your baby and especially important until your baby can make his or her own thyroid hormone.

Your thyroid makes a second type of hormone, T3. Early in pregnancy, T3 can’t enter your baby’s brain like T4 can. Instead, any T3 that your baby’s brain needs is made from T4. T3 is included in a lot of thyroid medicines made with animal thyroid, such as Armour Thyroid, but is not useful for your baby’s brain development. These medicines contain too much T3 and not enough T4, and should not be used during pregnancy. Experts recommend only using levothyroxine (T4) while you’re pregnant.

Some women with subclinical hypothyroidism—a mild form of the disease with no clear symptoms—may not need treatment.

If you had hypothyroidism before you became pregnant and are taking levothyroxine, you will probably need to increase your dose. Most thyroid specialists recommend taking two extra doses of thyroid medicine per week, starting right away. Contact your doctor as soon as you know you’re pregnant.

Your doctor will most likely test your thyroid hormone levels every 4 to 6 weeks for the first half of your pregnancy, and at least once after 30 weeks.1 You may need to adjust your dose a few times.


If you have thyroid problems, you can still have a healthy pregnancy and protect your baby’s health by having regular thyroid function tests and taking any medicines that your doctor prescribes.


Thyroid Disease and Eating During Pregnancy

What should I eat during pregnancy to help keep my thyroid and my baby’s thyroid working well?

Because the thyroid uses iodine to make thyroid hormone, iodine is an important mineral for you while you’re pregnant. During pregnancy, your baby gets iodine from your diet. You’ll need more iodine when you’re pregnant—about 250 micrograms a day. Good sources of iodine are dairy foods, seafood, eggs, meat, poultry, and iodized salt—salt with added iodine. Experts recommend taking a prenatal vitamin with 150 micrograms of iodine to make sure you’re getting enough, especially if you don’t use iodized salt. You also need more iodine while you’re breastfeeding since your baby gets iodine from breast milk. However, too much iodine from supplements such as seaweed can cause thyroid problems. Talk with your doctor about an eating plan that’s right for you and what supplements you should take.



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