Thyroid function in infants born to women with hypothyroidism: An observational study at an Australian tertiary perinatal centre

Objective: Concerns have been raised regarding thyroid dysfunction in infants born to women with hypothyroidism including those with autoimmune hypothyroidism. This concern has led to the practice of thyroid function testing in the early neonatal period. We evaluated the practice of performing a routine thyroid function test around 2 weeks of age in all healthy full-term infants (≥37 weeks gestation) born to women with hypothyroidism to identify thyroid dysfunction.

Design, patients, and measurements: This retrospective, observational single centre study included full-term infants born to women with hypothyroidism, including non-Graves' autoimmune hypothyroidism, over a 3-year period. Preterm infants and those born to women with Graves' disease or thyroidectomy were excluded.

Results: Of the 790 mother-infant dyads, 780 infants (99%) had normal thyroid function. Only 10 infants (1%) had thyroid stimulating hormone (TSH) levels > 10mIU/L at 2 weeks of age (range 10.25-106.37 mU/L). Of these, follow-up thyroid function normalized in nine infants within 2 weeks. A routine newborn screening test identified congenital hypothyroidism in one infant. No infant born to women with known presence of anti-thyroid antibodies had TSH levels > 10 mIU/L. Thyroid function was normal for most infants where maternal anti-thyroid antibodies were not known (125/133, 94%).

Conclusions: Infants born to women with hypothyroidism (including autoimmune hypothyroidism) had normal thyroid function in the early neonatal period. A small proportion of infants may develop TSH levels > 10 mU/L that normalizes by 4 weeks of age. The practice of routine thyroid function testing for this cohort in addition to newborn screening test offers no additional benefit.