Maternal hypothyroidism may affect fetal growth and neonatal thyroid function

To investigate the pituitary-thyroid axis function in the early neonatal period of newborns to hypothyroid mothers who have been apparently adequately treated.

Among the 27,386 full-term newborns delivered over a 6-year period, 259 were born to 250 treated hypothyroid mothers (0.9%); 246 of these newborns constituted the study group. Controls were 139 term healthy neonates from healthy group-matched mothers. The study infants and controls underwent thyroid function tests in a prospective design. A single blood sample was collected from each infant at 25-120 hours of life.

Compared with the controls, serum thyroid-stimulating hormone (TSH) levels were higher in the study neonates (P <.005), as were those of serum free thyroxine (T4) (P <.03), particularly at 49 hours of life or older (P <.001). At 49-120 hours, 44.7% of the study group newborns had serum free T4 levels greater than the 95th percentile of the controls (P <.001), and 16.8% had significantly higher TSH levels (P <.001). Serum free T4 correlated positively with TSH in the controls (r =.316) but not in the study newborns (r =.062, P =.36). Neonatal TSH at 49 hours or older correlated positively with maternal TSH during pregnancy in the 18 cases where maternal TSH values during pregnancy were available (r =.751, P <.001). Birth weight and head circumference were significantly lesser in the study group (P <.001).

The impaired intrauterine growth and the unduly elevated serum values of TSH and serum free T4 found in a substantial fraction of the study newborns might reflect an insufficient level of hormone replacement therapy of their hypothyroid mothers during pregnancy, despite an assumed adequate management. Gestational hypothyroidism requires close monitoring.