How Pregnancy Hormones Affect Your Thyroid

A pregnant woman speaking with her obstetrician

Pregnancy causes specific changes in hormone production in a woman's thyroid gland. High or low levels of thyroid hormones can have direct adverse effects on her health and on her baby's development, therefore diagnosis and treatment of thyroid disease is important during pregnancy.

The Role of Thyroid Hormones

Thyroid hormones are important to pregnant women and babies because they help to regulate metabolism and energy, as well as maintain vital systems in the body such as the heart, lungs and temperature regulation. They also have a major role in a baby's growth and development, especially the brain and nervous system.

Hormonal and other changes influence how the thyroid gland functions during pregnancy. Many women with hyperthyroidism (too much hormone) or hypothyroidism (too little hormone) are diagnosed before pregnancy, but changes of pregnancy can cause new thyroid disease to develop.

Effect of Pregnancy Hormones on Thyroid Function

Two hormones of pregnancy, Human Chorionic Gonadotropin (HCG) and estrogen affect the normal function of the thyroid gland and how the hormones are carried in the blood. The thyroid gland becomes more active and thyroid hormone production increases, transporting proteins in the blood. The result is that the level of thyroid hormone tends to be higher in pregnancy.

Normal Thyroid Hormone Production

The thyroid gland makes the hormones thyroxine (T4) and triiodothyronine (T3). The pituitary gland in the brain controls this production by secreting Thyroid Stimulating Hormone (TSH), which tells the thyroid when to make more or less hormones. In turn, if the level of thyroid hormones in the blood is low, this signals to the pituitary to make more TSH to stimulate the thyroid to increase hormone production.

On the other hand, if thyroid hormone levels are high, the feedback to the pituitary causes it to secrete less TSH so the thyroid can make less hormones. This continuing feedback keeps the level of thyroid hormones as well as TSH in a constant range in the blood.

Effect of HCG on Hormone Production

The hormone of pregnancy, Human Chorionic Gonadotropin (HCG) secreted by the early embryo and then the placenta, is similar to TSH and acts on the thyroid gland to make more thyroid hormones. Total thyroid hormone level (bound plus free) therefore is higher in pregnancy.

The increased stimulation by HCG of the thyroid gland causes it to get bigger, but usually not big enough to tell the difference on examination unless there is overstimulation of the gland. If the thyroid feels larger in pregnancy, this is abnormal.

Thryoxyine-binding globulin

Most of the thyroid hormones are carried in the blood by binding or transport proteins (globulins). The major hormone Thyroxine (T4) is carried in the blood by thyroxine-binding globulin and the free, active unbound portion is delivered to tissues. The free hormone is the portion that feeds back to tell the pituitary to regulate TSH, which in turn regulates the thyroid.

Effect of Estrogen on Binding of Thyroid Hormones

Higher estrogen levels during pregnancy cause an increase in thyroxin-binding globulin (TBG). Binding of more hormone leaves less of the active hormone, leading to an increased amount of the total thyroid hormones in the blood. The free amount of hormones remains normal.

Diagnosing Thyroid Disease in Pregnancy

Measuring blood levels of TSH and the thyroid hormones, bound and free, is the standard way to diagnose thyroid disease before and during pregnancy. The effects of HCG and estrogen on thyroid hormones makes it more difficult to interpret the tests and make an accurate diagnosis of hyperthyroidism or hypothyroidism during pregnancy unless a doctor understands these changes.


Hyperthyroidism happens when the thyroid gland makes more hormone than normal. Thyroid hormone and antibodies cross the placenta, and too much can affect the baby after delivery.

Common Causes in Pregnancy

  • Grave's Disease: This is the most common cause of hyperthyroidism in pregnancy. Grave's is an autoimmune disease in which the body makes antibodies against its own tissues. The thyroid antibody, thyroid-stimulating immunoglobulin (TSI), acts like TSH and stimulates the thyroid gland to make more thyroid hormone.
  • Inflammation of the thyroid: This causes leaking of thyroid hormones into the blood.
  • Prescription thyroid hormone: If the dose of prescription hormone used to treat hypothyroidism is too high, this can cause symptoms of hyperthyroidism.
  • Too much iodine in the diet: Too much iodine causes the gland to make more hormone.

Symptoms of Hyperthyroidism

Symptoms of hyperthyroidism include the following:

  • Unexplained weight loss
  • Not enough weight gain during pregnancy
  • Increased appetite
  • Bulging, watery, irritated eyes
  • Rapid heartbeat or irregular heartbeat
  • Shakiness or anxiety
  • Intolerance to heat
  • High blood pressure
  • Fatigue
  • Diarrhea
  • Hair loss
  • Enlargement and protrusion of the gland (goiter)
  • Severe nausea and vomiting (hyperemesis gravidarum)

Note that symptoms of fatigue, faster heartbeat and heat intolerance are also common symptoms of normal pregnancies.

Effect on the Mother and Baby

Women with Grave's disease may actually notice improvement in their symptoms because pregnancy suppresses the immune response of the body so it doesn't attack the baby as a foreigner. Sometimes women may need to reduce the dose of their anti-thyroid medicine during pregnancy. Symptoms may flare up, however, after delivery when the immune suppression reverses.

Women with hyperthyroidism from Grave's and their babies must be monitored closely. If the mother is not adequately treated or is undiagnosed, adverse effects include:

  • Miscarriage
  • Premature birth
  • Low birth weight
  • Preeclampsia, a rise in blood pressure that can have serious outcomes for mother and baby
  • Congestive heart failure
  • Thyroid storm, which is a worsening of symptoms that occur suddenly
  • TSI antibodies made before pregnancy and still present in the mother's blood can cross the placenta and stimulate and enlarge the baby's thyroid. The enlarged gland can compress the windpipe and cause difficulty with breathing after birth.
  • Because of a higher level of thyroid hormone after birth the baby can also have a rapid heartbeat that can cause heart failure, irritability, failure to gain weight and early closure of the soft spot in front of the skull.

Treatment of Hyperthyroidism in Pregnancy

Hyperthyroidism is treated with medicines to decrease hormone production or surgery to remove part of the gland. Medicines include methimazole (Tapazole, Northyx) or propylthiouracil (PTU). Doctors usually prescribe PTU in the first trimester then switch to methimazole because it is associated with uncommon fetal abnormalities. Treatment also includes radioactive iodine or removing the gland.


Hypothyroidism happens when the thyroid gland makes less thyroid hormones. A baby depends on his mother for thyroid hormone until his thyroid starts to make its own at around 12 weeks.

Common Causes in Pregnancy

  • Hashimoto's Thyroiditis: This is most common cause of hypothyroidism in pregnancy. Hashimoto's is also an autoimmune disease where the anti-thyroid antibodies attack thyroid cells, causing the gland to make less hormone.
  • Low iodine in diet: Iodine is important in thyroid hormone synthesis. Some women don't get enough in their diet, so the thyroid gland can't make enough hormone.
  • Viral infections such as a cold can sometimes be a cause.
  • Tumor in the pituitary can decrease TSH secretion.
  • Sheehan's syndrome: Severe bleeding after pregnancy can cause damage to the pituitary gland and it cannot make TSH.
  • Destruction of the thyroid gland: Hypothyroidism can occur after treatment of hyperthyroidism if the gland is destroyed from taking anti-thyroid medicines or if the gland was removed by surgery before pregnancy to treat the disease.

Subclinical Hypothyroidism

In subclinical (symptoms are absent) hypothyroidism a high TSH blood level indicates the disease is present but thyroid hormones are normal. It means that the thyroid gland is worker harder to maintain a normal hormone level. This occurs in about two to three percent of pregnancies.

Symptoms of Hypothyroidism

Common symptoms of hypothyroidism during pregnancy include the following:

  • Severe fatigue
  • Varying degrees of cold intolerance
  • Pale, dry skin
  • Hoarseness
  • Muscle cramping
  • Constipation
  • Difficulty with memory, concentration and focus
  • Swelling in arm and legs

Effect on the Mother and Baby

Untreated or under-treated hypothyroidism in pregnancy can cause the following problems:

  • Major effects on tissue growth and organ development, especially the brain and nervous system
  • Miscarriage
  • Stillbirth
  • Low birth weight
  • Preeclampsia
  • Anemia
  • Congestive heart failure, which is uncommon


Hypothyroidism is treated with prescription thyroxine pills taken daily. Women with hypothyroidism before pregnancy may need a higher dose of the replacement. It is important to ensure adequate replacement by checking blood levels of thyroid hormones every six to eight weeks.

Postpartum Thyroiditis

After delivery some women can develop a kind of inflammation of the thyroid (postpartum thyroiditis) that causes thyroid hormone to leak out of the glands in the thyroid into the blood. The increased hormone level in the blood cause symptoms of hyperthyroidism.

Six to twelve months later a woman may develop hypothyroidism because the gland can't make enough hormone. Sometimes the gland later recovers hormone production, but sometimes it does not and the woman has to be treated for permanent hypothyroidism.

Pre-Pregnancy Thyroid Disease

Women with hyperthyroidism or hypothyroidism diagnosed before pregnancy should discuss their disease with their doctor before getting pregnant. It might be helpful to get your thyroid tests done at least a month before trying to get pregnant to ensure that you are on the right dose of medicine. This will eliminate any effects on the baby during early development before his thyroid starts functioning, especially from hypothyroidism.

Review your thyroid medicines to make sure you are not taking anything that will harm your baby.

If you have a new diagnosis of thyroid disease, it might be better to wait at least three months to ensure that your treatment is effective before getting pregnant. Thyroid disease can also cause difficulty with ovulation and lead to irregular periods and make it more difficult to get pregnant.

Awareness and Screening

It is important to be aware of the importance of the effects of thyroid disease to both mother and baby and the effects of pregnancy on the thyroid gland. If thyroid disease is found, screening before or early in pregnancy and treatment by an expert will prevent illness in you and adverse outcomes for you and your baby.

Be aware of symptoms of thyroid disease such as fatigue or constipation, which some women may ignore because they are generalized symptoms that can also occur frequently throughout pregnancy.

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