Hypothyroidism During Pregnancy

by Dr. Michele Brown OB/GYN

Here's What You'll Find Below:  • Overview
  • Hypothyroidism in pregnancy
  • Dangers to mother and fetus
  • Causes of hypothyroidism
  • How is it diagnosed?
  • What is the treatment?
  • Subclinical hypothyroidism
  • How does hypothyroidism affect your baby?
  • Summing it up

feeling sickLet's pretend we have a 26-year-old, newly-pregnant woman sitting on the exam table in her obstetrician's office. While she is excited about her pregnancy, she is verbal and is telling her physician about the terrible nausea and vomiting, anxiety, difficulty sleeping, and general fatigue.

The obstetrician can easily see that she seems to be suffering the normal symptoms of pregnancy but is also a bit worried that it may be something else. Even if nausea and vomiting occur in 50–80% of all pregnant women, especially between the 5th and 13th week, a good obstetrician will go a bit further to make sure it is not something more serious before suggesting a medication, or a supplement, to reduce the symptoms of nausea and vomiting.

Is it possible that this woman is experiencing something more severe than the normal, early pregnancy symptoms? Could it be hypothyroidism?

Quick Review

One of the great masqueraders for pregnant women is thyroid disease. Many of the symptoms that women experience in the early stages of pregnancy are the exact symptoms that occur with thyroid problems. Women will commonly experience fatigue, weight gain, constipation, insomnia, and lethargy.

Health care providers will often reassure patients that this is normal and these symptoms are due to the hormonal and physiological changes that one expects with the early stages of a healthy pregnancy. However, one must be on the alert that these same symptoms could be representative of a much more serious underlying problem; one that could have major, negative ramifications on the pregnancy and the newborn infant.

Left undiagnosed and untreated, hypothyroidism (low thyroid hormone) could result in serious, high-risk conditions during the pregnancy. Prematurity, preeclampsia, placental separation (abruption), and/or serious consequences in the child such as congenital cretinism (mental retardation, deafness, muteness).

This article will focus only on hy-PO-thyroidism. (when you have too little thyroid hormone) and its effects on pregnancy.

Symptoms of hypothyroidism often mimic a normal early pregnancy, such as weight gain and lethargy with a decrease in exercise ability. Hypothyroidism is defined as the inability to manufacture thyroid hormone by the thyroid gland. Missing this diagnosis can have grave, irreversible neurological consequences for the fetus.

Hypothyroidism in Pregnancy

How often does hypothyroidism occur in pregnancy?

Hypothyroidism occurs in .1 to .3% of pregnancies. It can be associated with other autoimmune disorders such as diabetes. It is often a cause of difficulty in conceiving since women with this condition have difficulty ovulating.

What are the symptoms of hypothyroidism?

intellectual slowness
voice changes (hoarseness)
lethargy or decrease in exercise capacity
prolonged relaxation of deep tendon reflexes
concentration difficulties

intolerance to cold
muscle cramps
hair loss
dry skin
carpel tunnel syndrome
weight gain

Dangers to Mother & Fetus with Untreated Hypothyroidism

postpartum hemorrhage

low birth weight
placental abruption
intrauterine growth restriction
congenital cretinism (growth failure,
  mental retardation, deafness, muteness)

Causes of Hypothyroidism

The most common cause of hypothyroidism in pregnancy in the United States is Hashimoto's Thyroiditis. The body produces antibodies against the thyroid gland rendering it unable to manufacture the hormone. However, the most common cause of hypothyroidism world wide is iodine deficiency. Iodine is essential for the manufacture of the hormone.

Other causes include subacute thyroiditis (viral illness of the thyroid gland), certain drugs (ferrous sulfate, phenytoin, rifampin), pituitary or hypothalamic disease, or prior treatment with radioactive iodine to treat Graves Disease.

How is Hypothyroidism Diagnosed?

A blood test is used to detect hypothyroidism. Diagnosis in pregnancy is made by an elevated TSH (thyroid stimulating hormone which is made by the pituitary) and a corresponding low thyroid hormone level (T4).

One can also measure antibody levels in the thyroid hormones (antithyroglobulin, antithyroid peroxidase). Measurement of antibody levels is important because women who have antibodies are at increased risk of pregnancy complications and also increased risk of postpartum thyroid dysfunction. Women can have a goiter or large swelling in the neck area.

Having one autoimmune disease increases the chance of developing another. Women with type I diabetes have a 5 to 8% chance of developing hypothyroidism during pregnancy and a 25% chance of developing postpartum thyroid disease.

How is Hypothyroidism Treated?

The treatment of this disease is to replace the thyroid hormone with levothyroxine till the TSH levels are normal. Generally levels are followed each trimester of the pregnancy since the demands of pregnancy may necessitate an increase in dosage. If the cause of the hypothyroidism is due to iodine deficiency, iodine supplementation is essential not only during pregnancy but also after birth.

Women who take iron during pregnancy due to anemia will have difficulty absorbing their thyroid hormone so these medications should be spaced at least 4 hours apart.

What is Subclinical Hypothyroidism?

This is a subgroup of thyroid impairment found in 2–5% of pregnant women. Generally the T4 is normal but the TSH is elevated. Subclinical hypothyroidism has been linked to faulty placental development. There is a three fold increased risk of abruption, higher miscarriage rate, and a two fold increase in the incidence of preterm birth leading to impaired neurodevelopment in the child.

There is currently a great debate on whether women who present with this condition should be treated since studies have not shown a benefit with replacement. These patients should be followed after delivery because of the increased incidence of developing overt thyroid disease postpartum.

Women with Thyroid Disease Prior to Pregnancy

Pregnancy has a beneficial effect on women with preexisting thyroid disease. Due to the suppression of the immune system, the antibodies found in Hashimoto's disease decline but, immediately postpartum, there can be a resurgence with marked worsening of the condition. There can be a noticeable reduction in goiter size during the pregnancy.

How Does Hypothyroidism Affect your Baby?

Congenital hypothyroidism occurs in one in 4,000 births. There can be multiple etiologies from genetic, immunologic, environmental, and drug induced causes. It is critically important not to miss this diagnosis in the infant since developmental retardation can occur if the condition goes untreated.

Often infants appear normal at birth but deteriorate over several months. Infants can have severe retardation, deafness and muteness. This is the most common cause of mental retardation worldwide. If delay in treatment of congenital hypothyroidism is beyond 3 months, the chance of normal development is low.

Currently there is mass neonatal screening programs for all babies in all 50 states prior to leaving the hospital.


Hypothyroidism in pregnancy is a condition that should be recognized and treated so severe maternal and fetal complications can be avoided.

If thyroid disease exists prior to pregnancy, women should be followed closely and adjustments made to medication throughout the pregnancy. Care should be taken not to miss postpartum thyroid problems which can be transient but have a tendency to reoccur in subsequent pregnancies.

Thyroid dysfunction during pregnancy, both overt and sub-clinical, can predict later thyroid disease. There is also a corresponding six fold risk of diabetes later on in life.

On the other hand, most pregnant women and their babies will not experience significant problems if the hypothyroidism is mild to moderate and, if properly treated, the pregnancy can be expected to progress normally. When treatment is complete, most women feel much better than before their treatment and are able to do more and to enjoy the activities of their daily lives.

Dr. Brown, founder of Beauté de Maman, is a board-certified member of the American College of Obstetrics and Gynecology, a member of the American Medical Association, the Fairfield County Medical Association, Yale Obstetrical and Gynecological Society and the Women's Medical Association of Fairfield County. She is a magna cum laude graduate of Tufts University, completed her medical training at George Washington University Medical Center and completed her internship and residency in obstetrics and gynecology at Yale-New Haven Hospital. Dr. Brown has a busy obstetrical practice in Stamford, Connecticut and, as a clinical attending, actively teaches residents from Stamford Hospital and medical students from Columbia Presbyterian Hospital in New York.

Copyright © Michele Brown. Permission to republish granted to Pregnancy.org, LLC.