Hypothyroidism during Pregnancy

Pregnancy is a very stressful situation for every woman, and for her body. Lots of changes start to occur to prepare the mother’s body and to supply of nutrients and elements the new fetus and help with his development and growth. Biochemistry, physiology, and endocrine changes and disorders could happen since the beginning of the pregnancy up to the end; some of them will be auto-regulated for the own body, but other will lead to diseases or conditions such as hypothyroidism.

Hypothyroidism is defined as the low production of thyroid hormones by the thyroid gland. The thyroid hormones are triiodothyronine (T3) and thyroxine (T4). The thyroid-stimulating hormone (TSH), secreted by the anterior pituitary gland, regulates the production of T3 and T4. The thyroid hormone is necessary not only for the mother’s body but for the baby’s brain and nervous system development, too. The first 12 weeks, the fetus depends on the mother’s thyroid hormone that receives through the placenta, until his thyroid gland starts to function at around 12 weeks.

In the early stage, symptoms of hypothyroidism could be constipation, fatigue, increased sensitivity to cold temperature, dry skin, depression, feeling slowed down, joint or muscle pain, weakness, hair loss, and weight gain. Some of these symptoms are sometimes confused with symptoms related to the pregnancy, making difficult to think that the mother is having a problem. The thyroid will be enlarged slightly during pregnancy, but when the enlargement is noticeable, the thyroid function should be evaluated even when no symptoms give us a clue of thyroid disease. Other late symptoms of hypothyroidism are hoarseness, slow speech, decreased taste and smell, puffy face, low body temperature, and slow heart rate.

There are two pregnancy hormones -estrogens and human chorionic gonadotropin (hCG)- that can cause an increase in the levels of thyroid hormone. The hCG stimulates the thyroid to produce more thyroid hormone, and the estrogen increases the synthesis of thyroid-binding globulin (TBG). The thyroid hormone thyroxine (T4) is going to bind to TBG; as a result, total T4 levels will be elevated but with standard free T4 (unbound hormone).

Hypothyroidism is a problem present in 0.3 – 0.5% of pregnant women, but it is a disease that can be treated. Early diagnosis is the clue to treat the disorder and avoid the consequences in the fetus and the mother. Hypothyroidism in the pregnancy can lead to preeclampsia, anemia, babies with lower intelligence, low birth weight, miscarriage, respiratory distress in the newborn, and even congestive heart failure in rare cases.

Women with known hypothyroidism have fertility problems. If they conceive, there will be in a higher risk to abortion, gestational hypertension, abruptio placenta and hemorrhage during postpartum.

During pregnancy, the thyroid gland increases its size by 10% with increased iodine requirements and thyroxine production. Thyroid hormones must be produced in 50% more to supply of the hormone to the developing fetus and the mother. To prevent hypothyroidism due to low iodine intake, pregnant women, according to the World Health Organization, must get 250 micrograms of iodine each day, but because most of them won’t achieve this amount, a supplement of 150 micrograms taken daily by mouth is required.

Other causes of hypothyroidism include previous thyroid surgery, radioiodine ablation of the thyroid to treat thyroid cancer or hyperthyroidism, and medicines such as Rifampicin and phenytoin.

The most common cause of hypothyroidism during pregnancy is the autoimmune thyroiditis or Hashimoto's thyroiditis, an inflammatory disorder where the body recognizes the thyroid gland and thyroid hormones as a threat and the antibodies and white blood cells produced, will attack the thyroid gland tissue causing damage in the healthy thyroid cells. Some studies related the thyroid autoimmunity with a familiar pattern (dominant inheritance -when up to 50% of the first degree relatives have some autoimmune thyroiditis or thyroid antibodies in blood-). Women with autoimmune thyroid disease or those euthyroid with positive TPO antibodies (thyroid peroxidase) have higher miscarriage rates. Perinatal mortality and babies large for their gestational age have been reported from mothers with autoimmune thyroid disease.

Diagnosis of hypothyroidism during pregnancy

The diagnosis will be made through the TSH and T4 levels measurement. When symptoms are present, increased TSH and low levels of free T4 make the diagnosis of hypothyroidism. If during a routine screening, TSH is found in high levels (between 2.5 and 10 mIU/l), with normal free T4 and no symptoms, the diagnosis will be subclinical hypothyroidism. If TSH is above 10 mIU/l with normal or low T4, primary hypothyroidism must be considered. There are different opinions about if pregnant women without symptoms must be screened routinely, but the finding of subclinical hypothyroidism and its treatment can prevent maternal and fetus consequences. Another recommendation that is not yet universal is the determination of TPO antibodies to study the risk of miscarriage, especially in mothers with miscarriage history; but this test does not predict future outcome.

Hypothyroidism treatment in Pregnancy

The treatment it is focused on restore the thyroid hormone. Levothyroxine is the synthetic form of T4, and it is safe to use during the pregnancy. Levothyroxine must be administrated daily by mouth to women who get diagnosed with hypothyroidism during pregnancy, and women with subclinical hypothyroidism with or without symptoms. The goal is to reduce the risk of malformations or birth problems in the baby and other consequences to the mother. A mother who had hypothyroidism before the pregnancy must increase the dose of the synthetic hormone according to the doctor’s recommendation. TSH screening every 6 to 8 weeks will determine if the Levothyroxine dosage used is correct or if it needs to be modified.

Hypothyroidism during pregnancy is a condition that can occur with some frequency but can be easily diagnosed and treated. It is essential to make an early screening and physical thyroid exam to the expectant women and pay attention to the symptoms that can be developing. The early detection and treatment can help to reduce the risk of miscarriage, mother health problems, gestational problems to the mother, birth defects, or early death of the baby.

References:

1. Jameson, J. L., & De Groot, L. J. (2010). Endocrinology-E-Book: Adult and Pediatric. Elsevier Health Sciences.