Fetal free thyroxine concentrations in pregnant women with autoimmune thyroid disease.

CONTEXT: Fetuses from mothers with autoimmune thyroid disease (AITD) may be affected by antithyroid antibodies, antithyroid drugs, and iodine.

OBJECTIVE: The study correlated fetal free T(4) (fT4) with fetal ultrasound parameters and maternal thyroid function, thyroid antibodies, and medication dose from mothers with AITD.

DESIGN AND SETTING: The study was designed as a prospective cohort study and conducted in an academic referral center.

PATIENTS: Eighty-three of 85 women with AITD completed the study; 38 were treated for hyperthyroidism and 25 for hypothyroidism, and 20 were euthyroid.

MAIN OUTCOME MEASURES: Outcomes were as follows: 1) fetal-fT4, TSH, ultrasound parameters (morphology, biometrics, heart rate); and 2) maternal-fT4, TSH, antithyroid drug dose, and antithyroid antibodies, thyroid peroxidase and TSH receptor (TRAK). Parameters were determined at the same time, between the 22nd and 33rd wk gestation.

RESULTS: A total of 48.3% of fetuses from hyperthyroid mothers, 60% of fetuses from hypothyroid mothers, and 10% of fetuses from euthyroid mothers had elevated fT4 levels (P = 0.006). In hypothyroid mothers, the presence of both thyroid antibodies was related to fetal hyperthyroidism, whereas absence was related to fetal euthyroidism (P = 0.019). Hyperthyroid mothers (TRAK-positive, thyroid peroxidase-negative) with hyperthyroid fetuses had significantly higher mean TRAK than hyperthyroid mothers with euthyroid fetuses (13.7 vs. 3.7 IU/liter; P = 0.02). Fetal fT4 correlated weakly negatively with maternal TSH within the normal range, but not with ultrasound parameters or with antithyroid drug dose.

CONCLUSION: High fetal fT4 levels were unexpectedly frequent in women with AITD, including maternal autoimmune hypo- and hyperthyroidism. Further studies are needed, as well as noninvasive methods to assess fetal thyroid function.