Thyroid function in mothers of hypothyroid newborns.

OBJECTIVE: To assess etiologic significance of maternal autoimmune thyroid disease on incidence of transient congenital hypothyroidism in newborns.

METHODS: Thyroid function was assessed in 259 mothers of hypothyroid newborns from the Quebec newborn screening program between 1984 and 1996. Maternal blood samples were taken routinely at the time of follow-up serum sampling of the infants, for measurements of free thyroxine, TSH, and antimicrosomal autoantibodies. Thyroid scans were conducted to characterize the type of thyroid dysfunction in affected infants. Results were compared with similar data from a control population of 1773 pregnant women recruited from Quebec City between 1991 and 1992. These samples were collected at routine second-trimester prenatal screenings.

RESULTS: The mothers of congenitally hypothyroid infants had higher prevalence of suppressed serum TSH concentrations compared with the control population (6.1% versus 2.9%, P = .01), higher prevalence of increased serum TSH levels (7.0% versus 0.9%, P < .001), and similar prevalence of positive antimicrosomal antibodies (11.6% versus 12.1%, P = .90). The prevalence of transient congenital hypothyroidism in the study population of hypothyroid infants was 27%, which is significantly greater than the 15% observed in our overall population of 523 congenitally hypothyroid infants studied between 1974 and 1996 (P = .04). Antimicrosomal antibodies were demonstrable in ten (77%) of 13 mothers of infants with transient congenital hypothyroidism, and antimicrosomal antibodies or abnormal serum TSH levels in 13 of 13 (100%).

CONCLUSION: These results indicate that most, if not all, cases of transient congenital hypothyroidism were attributable to maternal autoimmune thyroid disease, and presumably to maternal TSH receptor-blocking antibody. They suggest that maternal autoimmune thyroid disease is associated with much higher prevalence of transient congenital hypothyroidism than suspected. Because of the 20% prevalence of autoimmune thyroid disease and thyroid dysfunction in the mothers of congenitally hypothyroid infants, we recommend thyroid assessment of such women, if this information is not available.