As in the general population, the most common cause of hyperthyroidism in pregnancy is Graves' disease. But diagnosing hyperthyroidism can be tricky. Pregnant women cannot have an RAIU and scan because the radioactivity could potentially affect the baby's developing thyroid gland.
But untreated and active Graves' disease poses serious risks to both the mother and the baby. In the mother, the risks include:
Pre- eclampsia, high blood pressure and protein in the urine that can lead to kidney damage
In the baby, a mother's active Graves' disease can raise the risk for several problems, including:
Fetal tachycardia (rapid heartbeat)
Low birth weight
Treating Hyperthyroidism in Pregnancy
Unless you have a mild case of hyperthyroidism that is causing no symptoms, not treating hyperthyroidism is simply not an option. The risks associated with untreated hyperthyroidism are high and outweigh any risks associated with the use of treatment. Still, treatment for this condition can be more complicated during pregnancy since your options are more limited.
Many moms experience morning sickness in their first trimester. But if it lingers, you may be suffering from higher than normal levels of HCG, which can cause severe morning sickness and trigger the overproduction of thyroid hormone, a condition known as transient hyperthyroidism. To treat this condition, the pregnant mom is usually given a beta-blocker for a few weeks.
Like any case of hyperthyroidism, the goal of treatment is to lower the amount of thyroid hormone. Most often, the therapy of choice is PTU, which is considered a safer option in pregnancy than the other antithyroid drug, methimazole (Tapazole).
Most doctors will aim to get your free T4 and free T3 levels into the high-normal range on the lowest possible dose of PTU. But establishing the right dose can be tough. Too much PTU can affect the baby and cause hypothyroidism and goiter. It can also trigger hypothyroidism in the mother.
And yet, you need enough antithyroid medication to reduce the amount of thyroid hormone circulating in the mom. That's why treatment always involves routine monitoring to make sure TSH and thyroid hormone levels are close to normal. Given the choice, most doctors would prefer pregnant patients to be slightly hyperthyroid rather than slightly hypothyroid. If you're hypothyroid, the risk to your baby is greater.
If a pregnant woman is allergic to or has a reaction to PTU, she may be given methimazole instead. If neither drug can be tolerated, you may require surgery to remove the overactive thyroid. Because any kind of surgery comes with inherent risks, it is not commonly done in pregnant women. RAI treatment (and scans) is never used in pregnant women because the radioactivity can destroy the baby's developing thyroid and cause permanent hypothyroidism.
To combat the symptoms of hyperthyroidism, your doctor may prescribe a beta-blocker such as atenolol. But these drugs are generally used with caution. Taken in excess, they can impair fetal growth and cause low birth weight.
Hyperthyroidism and Your Baby
Even if you're successfully treated for hyperthyroidism, and you take the smallest dose of medication possible, your developing baby is still at risk for a condition called neonatal thyrotoxicosis, which is basically hyperthyroidism. This condition is most likely to occur in women whose hyperthyroidism is caused by Graves' disease.
As you might recall, Graves' disease is accompanied by the presence of TSIs. Antithyroid medications and surgery do not destroy TSIs, which continue to linger in your blood and can transfer to your baby. In very rare cases, a developing baby may develop thyrotoxicosis while in utero. This condition tends to occur in women with Graves' disease who have very high levels of TSI. Intrauterine thyrotoxicosis can cause fetal tachycardia, failure to grow, advanced aging of the bones, and occasionally fetal death. Babies whose moms have elevated TSI should be carefully monitored before and after delivery.
Some developing babies whose mothers are taking PTU (or methimazole) may also develop hypothyroidism, especially if the dose is too high. An astute obstetrician may perform an ultrasound to determine if the fetus has a goiter. In that case, your dose may be reduced.
Although the numbers of babies of mothers with Graves' disease who become hyperthyroid is small, it's essential to detect it early since the condition is potentially fatal. For that reason, if you have hyperthyroidism, you should have your baby's thyroid tested at birth and soon after with your pediatrician.