Managing Pregnancy in Women With Thyroid Problems

Dose adjustments may be needed to ensure fetal health

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Your thyroid gland is important during pregnancy as it regulates the production of hormones called triiodothyronine (T3) and thyroxine (T4), each of which plays a critical role in the development of the baby’s brain and nervous system.

During the first trimester, the fetus depends on the mother’s supply of thyroid hormone, which is delivered through the placenta. In order to meet this need, the mother's thyroid production will typically go into overdrive, resulting in an enlargement of the gland itself.

This is usually not noticed and won't complicate the pregnancy in any way.

However, in some cases, the enlargement may be noticeable on clinical examination and suggest the woman has hypothyroidism, a condition where the thyroid gland isn't producing enough thyroid hormone.

Changes to Thyroid Function

Even under normal circumstances, pregnancy places stress on the thyroid gland. The need to increase hormone production by upward of 50 percent will typically result in the enlargement of the gland itself. In women with normal thyroid function, the gland may increase by 10 percent in size. In women with hypothyroidism, it may increase by anywhere from 20 to 40 percent.

Beyond the physical enlargement of the gland itself, there are changes in hormone production which doctors can monitor using blood tests. Chief among them is the TSH test, which measures the level of thyroid-stimulating hormone (TSH) in the blood.

TSH is the hormone produced by the pituitary gland that triggers the production of T3 and T4.

Because normal thyroid function is different during pregnancy, TSH values will change as the mother progresses from the first to third trimester. Under normal circumstances, the normal TSH value would range from 0.2 to 4.0 mlU/L.

If for any reason the thyroid gland cannot keep up during pregnancy, the value will drop, indicating a hypothyroid state. In such case, thyroid hormone replacement medication will be prescribed to replace the missing thyroid hormone. The mother will then be routinely monitored to assess TSH values, adjusting treatment as needed.

Monitoring TSH

Many laboratories establish their own trimester-specific reference ranges for TSH outlining what they consider to be "normal" values during each stage of pregnancy. If not, the American Thyroid Association (ATA) recommends the use of the following ranges:

  • first trimester: 0.1-2.5 mIU/L
  • second trimester: 0.2-3.0 mIU/L
  • third trimester: 0.3-3.0 mIU/L

If you have thyroid disease, you should be regularly monitored throughout the entire your pregnancy. If thyroid disease runs in your family or you have symptoms of the disease, it is important to inform your doctor so that you can be properly monitored and treated.

Hypothyroidism

If you are hypothyroid, it is crucial that you be treated both before and during your pregnancy. If left untreated or insufficiently treated, your hypothyroidism can cause developmental and motor problems in your child.

If you being treated for hypothyroidism, don't assume that you can continue managing your condition in the same way.

You may, in fact, need to increase your dosage of thyroid hormone replacement drugs by as much as 50 percent as soon as pregnancy is confirmed. Research suggests that 50 to 80 percent of hypothyroid women will need to do this.

According to the ATA guidelines, these increases should begin as early as weeks 4 to 6 and continue through to weeks 16 to 20 (after which it will typically plateau until delivery). Thyroid tests will need to be run every four weeks during the first half of pregnancy and then again between weeks 26 and 32.

Following delivery, medication doses will need to be reduced to pre-pregnancy levels with follow-up monitoring performed six weeks after the delivery date.

Graves' Disease

In some cases, a woman may experience an overactive rather than underactive thyroid. This is known as hyperthyroidism, which is frequently caused by Graves' disease.

If left untreated, hyperthyroidism can lead to a premature birth or preeclampsia (a pregnancy complication characterized by high blood pressure and organ damage). Risks to the baby include a low birth weight, a rapid heart rate, congenital defects, and other health concerns. In more severe cases, a stillbirth can occur.

Additionally, if you are pregnant and have Graves' disease, you are at an increased risk of developing a severe form of hyperthyroidism called thyroid storm. Also known as thyrotoxic crisis, it is caused by the excessive release of thyroid hormones which causes a potentially deadly increase in blood pressure, body temperature, and heart rate.

During pregnancy, Graves disease is typically treated with an antithyroid medication such as propylthiouracil during the first trimester and another called methimazole for the remainder of pregnancy.

Hashimoto's Disease

Hashimoto's disease, also known as Hashimoto's thyroiditis, is an autoimmune disease which attacks and gradually destroys the thyroid gland. Hypothyroidism is commonly outcome of the disorder and is treated in the same manner using hormone replacement therapy. 

Treatment of Hashimoto's disease during pregnancy involves treating the hypothyroidism, although additional attention should be made to keeping the TSH under 2.5 mlU/L as higher levels are associated with a two-fold increase in the risk of miscarriage.

A Word From Verywell

One important thing to remember is that if you are hypothyroid and you are actively planning to conceive, you need to talk with your doctor about adjusting your dosage of thyroid hormone replacement medication in order to optimize fertility. The goal is to maintain your TSH level below 2.5 mIU/L.

You should also work with your practitioner to confirm your pregnancy as early as possible, and have a plan in place to increase your dosage of medication by a predetermined amount as soon as your pregnancy is confirmed.

Source:

Alexander, E,; Pearce, E.; Brent, G.; et. al. “2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease during Pregnancy and the Postpartum.” Thyroid. 2017; 27(3):315-389.

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