Pregnancy and thyroid
During pregnancy, thyroid can be very dangerous for the mother and the baby, but it can be easily controlled with the use of appropriate drugs and effective treatment methods. Studies have shown that more than 4% of pregnancies are associated with thyroid disorders.
How to diagnose thyroid disorders during pregnancy?
Most pregnant women with hyperthyroidism are known to have had thyroid disease before the onset of gestation and will already be receiving treatment. A new diagnosis of hyperthyroidism is uncommon in early pregnancy, as untreated disease is associated with reduced fertility. However, in a series of 14 970 first trimester blood samples, undiagnosed Graves’ hyperthyroidism was present in about 0.15%.7 Features such as tachycardia, palpitations, systolic murmur, bowel disturbance, emotional upset, and heat intolerance may be seen in normal pregnancy but should alert the clinician to the possibility of hyperthyroidism, particularly if a goitre or more specific feature of thyroid disease (weight loss, eye signs, tremor or pre-tibial myxoedema) is observed. Newly diagnosed hyperthyroidism should be aggressively treated.
What is the interaction between maternal and infant thyroid function?
For the first 18-20 weeks of pregnancy, the baby is completely dependent on the mother for the production of thyroid hormone. By mid-pregnancy, the baby’s thyroid begins to produce thyroid hormone on its own. The baby, however, remains dependent on the mother for ingestion of adequate amounts of iodine, which is essential to make the thyroid hormones. The World Health Organization recommends iodine intake of 250 micrograms/day during pregnancy to maintain adequate thyroid hormone production. Because iodine intakes in pregnancy are currently low in the United States, the ATA recommends that US women who are planning pregnancy, pregnant, or breastfeeding should take a daily supplement containing 150 mcg of iodine.
Treatment of hyperthyroidism in pregnancy
Mild hyperthyroidism (slightly elevated thyroid hormone levels, minimal symptoms) often is monitored closely without therapy as long as both the mother and the baby are doing well. When hyperthyroidism is severe enough to require therapy, anti-thyroid medications are the treatment of choice, with PTU being preferred in the first trimester. The goal of therapy is to keep the mother’s free T4 in the high-normal to mildly elevated range on the lowest dose of antithyroid medication. Addition of levothyroxine to ATDs (“block-and-replace”) is not recommended. Targeting this range of free hormone levels will minimize the risk to the baby of developing hypothyroidism or goiter. Maternal hypothyroidism should be avoided. Therapy should be closely monitored during pregnancy. This is typically done by following thyroid function tests (TSH and thyroid hormone levels) monthly.
In patients who cannot be adequately treated with anti-thyroid medications (i.e. those who develop an allergic reaction to the drugs), surgery is an acceptable alternative. Surgical removal of the thyroid gland is safest in the second trimester.
Radioiodine is contraindicated to treat hyperthyroidism during pregnancy since it readily crosses the placenta and is taken up by the baby’s thyroid gland. This can cause destruction of the gland and result in permanent hypothyroidism.
Beta-blockers can be used during pregnancy to help treat significant palpitations and tremor due to hyperthyroidism. They should be used sparingly due to reports of impaired fetal growth associated with long-term use of these medications. Typically, these drugs are only required until the hyperthyroidism is controlled with anti-thyroid medications.
Treatment of hyperthyroidism in breastfeeding
You may have this question. Can a mother with Graves’ disease who is treated with anti-thyroid drugs breastfeed her baby?
The answer is yes. Although very small quantities of both PTU and methimazole are transferred into breast milk, total daily doses of up to 20mg methimazole or 450mg PTU are considered safe and monitoring of the breastfed infants’ thyroid status is not required.
References
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2270981/
https://www.ijrcog.org/index.php/ijrcog/article/viewFile/486/457/
