How can doctors detect and treat thyroid disease in a fetus before birth?

Details

This clinical overview discusses fetal thyroid disease, drawing on a reported case and previous studies rather than describing a new study with a specified design. It opens with a report in the New England Journal of Medicine of successful diagnosis and treatment of fetal goitrous hypothyroidism in a mother with Graves' disease. The fetus is increasingly being recognized as a patient in its own right in thyroid disease.

The fetal thyroid system develops independently of the normal maternal thyroid axis. Feedback suppression of thyroid-stimulating hormone (TSH) by thyroxine (T4) has been demonstrated in a 35-week fetus. Findings from congenital hypothyroidism suggest that a lack of fetal thyroid hormones may harm the developing fetal brain, including a lack of normal myelination. However, the gestational age at which thyroid hormone deficiency begins to adversely affect the fetus and developing central nervous system is uncertain.

Because congenital hypothyroidism is sporadic and, at the time described, there was no easy way to screen all pregnancies for hypothyroidism, fetal diagnosis and treatment had focused on pregnancies in which fetal hypothyroidism was suspected: when ultrasonography detected a fetal goiter, or when the mother had hyperthyroidism and might be taking antithyroid therapy. Intraamniotic injections of L-thyroxine had proven successful for fetal treatment. TSH in amniotic fluid might prove useful for diagnosing and treating a hypothyroid fetus. Previous studies had suggested that the fetal central nervous system's dependence on thyroxine was predominantly limited to the last 4–8 weeks of gestation.

Fetal hyperthyroidism may occur when thyroid-stimulating immunoglobulins cross the placenta in a mother with a history of hyperthyroidism due to Graves' disease. Fetal hyperthyroidism had been inferred from fetal tachycardia occurring as early as 25–30 weeks. Cardiomegaly, pulmonary hypertension, growth retardation, advanced bone age, craniosynostosis, prematurity, and fetal death had been described as consequences of intrauterine thyrotoxicosis. Fetal treatment using oral maternal PTU had been empiric.

The overview presents direct fetal blood sampling as a technique that would aid diagnosis and guide specific fetal treatment. It would allow confirmation of fetal thyroid abnormalities and direct treatment, rather than requiring fetal thyroid status to be inferred from less exact clinical parameters.