Fetal goiter, or thyromegaly, is a diffuse enlargement of the fetal thyroid gland. Goiter in the fetus can occur as part of a hypothyroid, hyperthyroid, or euthyroid state. Hypothyroid fetal goiters are more common than those associated with either the hyperthyroid or euthyroid states.

Goiter associated with fetal hypothyroidism can be caused by transplacental passage of antithyroid medications being used by a mother with hyperthyroidism, iodine deficiency, iodine intoxication, transplacental passage of antithyroid antibodies, congenital metabolic disorders of thyroid hormone synthesis, or hypothalamic-pituitary hypothroidism (Weiner et al., 1980; Romero et al., 1988; Noia et al., 1992; Soliman et al., 1994; Van Loon et al., 1995; Bruner and Dellinger, 1997; Vicens-Calvet et al., 1998). Most cases of hypothyroid goiter will not become apparent until the neonatal period. When goiter associated with hypothyroidism is recognized in utero it is most likely to be secondary to transplacental passage of drugs or congenital dyshormonogenesis because of an inherited enzymatic deficiency (Romero et al., 1988). Congenital hypothyroidism is a serious condition, which if not treated within the first 3 months of life is likely to result in irreversible mental retardation. Although newborn screening programs for congenital hypothyroidism have been extremely successful in reducing the incidence of mental retardation associated with this condition, it is possible that the presence of a hypothyroid state in utero may result in some degree of hearing, speech, and other intellectual impairment, despite early neonatal therapy (Rovet et al., 1987). This emphasizes the importance of thorough evaluation and treatment of fetal goiter whenever it is suspected prenatally.

Goiter associated with fetal hyperthyroidism is almost always caused by transplacental passage of a thyroid-stimulating IgG antibody from the mother (Wenstrom et al., 1990; Belfar et al., 1991; Hatjis, 1993; Hadi and Strickland, 1995; Heckel et al., 1997). Such antibodies are present in at least 95% of women with Graves disease. Thyroid-stimulating antibody levels in Graves disease may not reflect maternal thyroid status, as they are detectable in women who are clinically hyperthyroid, euthyroid, or hypothyroid. Therefore, whenever a patient with a history of Graves disease becomes pregnant, the fetus is at risk for hyperthyroid goiter, irrespective of the mother’s clinical thyroid state. The fetal thyroid gland becomes fully responsive to thyroid-stimulating substances, such as maternal IgG, only in the second trimester, so the detection of a fetal goiter before 20 to 24 weeks of gestation is unlikely.

Fetal goiter is extremely rare, with no published series sufficient to allow an estimation of its incidence. Hypothyroid goiter is more common than hyperthyroid goiter. Congenital hypothyroidism has an incidence of 1 in 4000 livebirths. Only a small fraction of these will be complicated by fetal goiter, because in the vast majority of cases goiter becomes clinically apparent only during neonatal life (Fisher et al., 1979). Inborn errors of thyroid hormone biosynthesis that result in congenital dyshormonogenesis and cause hypothyroid fetal goiter are rare, being present in 1 in 30,000 livebirths. Graves disease is seen in approximately 1% of pregnant women, and hyperthyroidism or hypothyroidism will develop in 2% to 12% of these fetuses or neonates (Hatjis, 1993). However, only a small fraction of this number of at-risk fetuses will be diagnosed prenatally with a fetal goiter.

The typical sonographic appearance of a fetal goiter is a symmetric, homogeneous mass in the anterior neck, with an echogenic consistency, and with some lobulation (Figure 33-1) (Heckel et al., 1997). The diagnosis is rarely made prior to 24 weeks of gestation. The fetal neck is often maintained in a state of hyperextension (Figure 33-2), and the trachea and esophagus may be compressed or displaced. Later in gestation, this compression may be associated with polyhydramnios, as swallowing becomes more difficult (Abuhamad et al., 1995). Color Doppler sonography can be used to aid in the diagnosis of a fetal goiter. The presence of a high flow pattern helps to confirm the diagnosis of goiter but does not necessarily imply a hyperthyroid state, as hypothyroid fetal goiters may also have increased vascularity (Soliman et al., 1994). Color Doppler sonography may also be useful in monitoring the response of a hyperthyroid fetus to antithyroid therapy, with a decrease in vascularity accompanying the resolution of the hyperthyroid state (Luton et al., 1997).

Sonographic screening for fetal goiter may be difficult because of problems in accurately identifying the fetal thyroid during early gestation. Nevertheless, normograms have been published for thyroid volume across a range of gestational ages, including transverse width and circumference of the gland, which may be helpful in diagnosing fetal goiter in patients at high risk for fetal thyroid dysfunction (Bromley et al., 1992).

After the sonographic diagnosis of a fetal goiter, a careful sonographic survey should be performed for other features suggestive of hyperthyroidism or hypothyroidism. Sonographic signs suggestive of fetal hyperthyroidism include cardiac hypertrophy, fetal tachycardia, hydrops, intrauterine growth restriction, advanced bone age with craniosynostoses, and hepatosplenomegaly. Sonographic signs of fetal hypothyroidism include cardiomegaly and fetal heart block. However, none of these additional sonographic features are sufficiently specific to reliably predict the fetal thyroid status. Magnetic resonance imaging (Figure 33-3) and 3D ultrasound imaging may help confirm the diagnosis (Figure 33-4) (Karabulut et al., 2002; Nath et al., 2005).