Neonatal hyperthyroidism

Infants of mothers with prior or current Graves disease have a low risk (around 1 to 2%) of developing neonatal hyperthyroidism, which is caused by transplacental passage of thyroid stimulating hormone (TSH)-receptor antibodies. Neonatal hyperthyroidism can cause heart failure in the neonate; if not recognised and managed early, the neonate is at increased risk of morbidity and mortality. Maternal thyroid ablation (radioiodine or thyroidectomy) before pregnancy does not eliminate the risk of neonatal hyperthyroidism, because TSH-receptor antibody can persist after ablation.

Neonatal hyperthyroidism almost always occurs in the context of elevated maternal concentrations of TSH-receptor antibody. To identify neonates at the highest risk, monitor maternal TSH-receptor antibody in all pregnant women with a history of Graves disease (including women who had ablative treatment), or who are currently taking an antithyroid drug for Graves disease. Measure TSH-receptor antibody in the first trimester; if the concentration is elevated, repeat the measurement at around 20 and 32 weeks gestation to give some indication of neonatal risk. However, a negative result does not preclude development of neonatal hyperthyroidism—not all assays can detect maternal antibodies that are biologically active in the fetus.

All neonates at risk of hyperthyroidism require measurement of thyroid function in the early days of life, and expert paediatric assessment. Measure serum TSH, free triiodothyronine (T₃) and free thyroxine (T₄) concentrations at delivery and again at day 2 to 7 of life. For neonates of mothers who were taking an antithyroid drug during pregnancy, further testing is recommended at day 10 to 14 of life; transplacental passage of the drug can delay the presentation of hyperthyroidism until the drug is cleared from neonatal circulation.

Neonatal hyperthyroidism resolves spontaneously, usually within a few weeks, but it can persist for 3 months or more. Short-term treatment with a beta blocker and an antithyroid drug may be needed; frequent assessment is required as the infant can become hypothyroid quickly if antithyroid treatment is continued after resolution.