Fetal free thyroxine concentrations in pregnant women with autoimmune thyroid disease

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

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.

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

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

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.

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.

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.