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Hyperthyroidism in Pregnancy and Neonatal Hyperthyroidism

Implications for Newborns

By

Updated June 10, 2014

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When a woman is hyperthyroid during pregnancy, there is a small risk -- approximately 1% to 2% -- that her baby will develop hyperthyroidism before birth -- a condition known as fetal thyrotoxicosis -- or be born with hyperthyroidism -- known as neonatal hyperthyroidism. And while it's rare, hyperthyroidism can also occur in the newborns of mothers who have a history of Graves' disease, and who have already been treated with thyroidectomy or radioactive iodine.

The reason these babies are at risk is that antibodies in the mother's bloodstream, called thyrotropin receptor-stimulating antibodies (TSHR-SAb), can cross the placenta, and cause an overactive thyroid in the fetus. The higher these antibody levels in the mother, the higher the risk of hyperthyroidism in the unborn baby or newborn.

What are the Signs and Symptoms of Neonatal Hyperthyroidism?

The signs and symptoms of hyperthyroidism in a newborn include the following:
  • An unusually small head circumference
  • An unusually prominent forehead
  • A dangerous accumulation of fluid (known as fetal hydrops)
  • Enlarged liver and/or spleen
  • Low birth weight
  • Premature birth
  • Warm, moist skin
  • High blood pressure
  • Fast heartbeat
  • Irregular heart rhythms
  • Irritability, hyperactivity, restlessness, poor sleep
  • Enlarged thyroid (goiter)
  • Difficulty breathing due to goiter pressing on the windpipe
  • Excessive or normal appetite, with poor weight gain
  • Bulging eyes, stare
  • Vomiting
  • Diarrhea

When Will Hyperthyroidism Symptoms Appear in a Baby?

Some babies are born hyperthyroid; others take days or even weeks to develop hyperthyroidism.

When the mother is not receiving antithyroid drugs, an infant can be born hyperthyroid. When the mother has normal thyroid function but was previously treated for hyperthyroidism (and still has circulating TSHR-SAb during pregnancy), the infant can also be born hyperthyroid.

In women receiving antithyroid drug treatment, the infants may be born with normal thyroid levels, but become hyperthyroid a few days after birth. Hyperthyroidism can actually show up in these babies as long as three weeks after birth. The hyperthyroidism develops as the antithyroid drug the mother was taking -- which was passing through the placenta to the baby -- wears off after birth.

What Monitoring is Necessary During Pregnancy?

For women who have Graves' disease/hyperthyroidism, regular fetal monitoring during pregnancy is essential. The baby's heart rate should be monitored; fetal thyrotoxicosis is suggested when the heart rate exceeds 160 beats per minute. Regular ultrasounds to evaluate fetal thyroid size are also important, as they can detect growth retardation, bone changes, and enlargement of the thyroid (goiter) -- all possible symptoms of fetal hyperthyroidism.

During the third trimester, the TSHR-SAb antibody levels should also be measured in all women with past or current Graves' disease. If the levels are elevated, this may help in predicting whether a newborn may be affected.

How is Hyperthyroidism in a Fetus or Newborn Diagnosed and Treated?

As noted earlier, elevated heart rate, thyroid enlargement in the fetus, and evidence of growth problems are all used to diagnose hyperthyroidism in a fetus. If a fetus is determined to be hyperthyroid, antithyroid drugs are often prescribed for the mother -- whether or not she is hyperthyroid herself -- to help normalize the baby's thyroid function.

Infants suspected of hyperthyroidism -- and especially babies born to mothers who have Graves' hyperthyroidism -- should have free thyroxine (Free T4) and thyroid stimulating hormone (TSH) levels measured at delivery or soon afterwards.

When neonatal hyperthyroidism is confirmed, treatment should be initiated promptly. I consulted UpToDate, the trusted online medical reference resource used by many physicians, to identify the appropriate therapies, which include the following:

  • An antithyroid drug, either propylthiouracil (PTU, 5-10 mg/kg per day) or methimazole (MMI, 0.5 to 1.0 mg/kg per day), should be administered every eight hours. I prefer PTU because it is easier to make small dose adjustments.
  • A beta blocker, such as propranolol (2 mg/kg per day every eight hours) is an important adjunct in controlling neuromuscular and cardiovascular hyperactivity.
  • Iodine, in the form of one drop (8 mg) of Lugol's solution (126 mg iodine/mL) every eight hours orally or SSKI (potassium iodide) one to two drops daily, can be given to inhibit thyroid hormone release.
  • Glucocorticoids can also be given in extremely ill infants. In addition to their antiinflammatory actions, glucocorticoids inhibit thyroid hormone secretion and decrease peripheral conversion of T4 to triiodothyronine (T3). Digoxin may be helpful if congestive heart failure is present.
Once improvement is evident, treatment should be gradually decreased and then discontinued. This may require frequent, ie, weekly, monitoring of thyroid function tests.
In a newborn with hyperthyroidism, a pediatric endocrinologist will typically be involved in helping to determine which of the treatment options are pursued.

Neonatal hyperthyroidism usually disappears on its own between 3 and 12 weeks of life, as the mother's antibodies disappear from the infant's circulation. There are some cases, however, where hyperthyroidism has continued for as long as six months or more in an infant.

A Special Note for Pregnant Women

If you have a history of Graves' disease, or you are hyperthyroid during pregnancy, it's essential to make all your healthcare providers -- including hospital personnel -- are aware of your hyperthyroidism, and the risks to your baby. Otherwise, this important medical information -- and the rare but possible scenario of newborn hyperthyroidism -- may get forgotten in the excitement of childbirth.

Want to learn more? See UpToDate's topic, "Evaluation and management of neonatal Graves' disease," for additional in-depth, current and unbiased medical information on name the condition/disease of relevance, including expert physician recommendations.

Source:

LaFranchi, Stephen. "Evaluation and management of neonatal Graves' disease." UpToDate. Accessed: February 2009.

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