The Fetal Thyroid: Normal and Abnormal Sonographic Measurements Bryann Bromley MD,* Fredric D. Frigoletto, Jr, MD,* Daniel Cramer, MD,* Rapin Osathanondh, MD, • Beryl R. Benacerraf, MD*t

The thyroid of 31 fetuses at low risk for perinatal thyroid disease were evaluated sonographically. The transverse width and circumference of the fetal thyroid was measured prospectively to provide normative val ~ ues for each gestational age. In addition, the thyroid of 23 fetuses at risk for thyroid disease were examined sonographically and compared to the control group. At birth, 18 of the neonates had no evidence of thyroid dysfunction, whereas 5 newborns had goiters and ab-

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omen with a history of Graves' disease have an increased risk for fetal thyroid dysfunc~ tion. The fetal thyroid can be imaged as early as the middle of the second trimester, and fetal goiters have been identified in utero. 1 · ~ Prenatal care for women with hyperthyroidism includes surveillance of the fetus for the development of complication!i associated with fetal thyroid disease, such as intrauterine growth retardation, oligohydramnios, tachycardia, and fetal thyroid enlargement. 3 Evaluation of the fetal thyroid as well as surrounding neck structures is an important part of the sonographic evaluation in these patients, as even small neck masses may compress the airway and result in respiratory distress in the newborn. Although gross fetal goiters have been described sonographically, to our knowledge there are no published nomograms for fetal thyroid sizes at various gestational ages. In this study, we measured thyroid size in fetuses at no known risk for thyroid disease and Received March 26, 1991, from the Departments of • Obstetrics and Gynecology and tRadiology. Brigham & Women's Hospital, Harvard Medical School, Boston. Massachusetts. Revised manuscript accepted for publication june 12, 1991. Address correspondence and reprint requests to Dr. Bryann Brom· ley: Diagnostic Ultrasound Associates, 333 Longwood Avenue, Bos· ton, MA 02115.

normal thyroid function. The fetal thyroid measurements for these 5 neonates were above the upper limit of the 95% confidence interval compared to the control group. The other 18 fetuses in the group at risk for thyroid disease but without evidence of thyroid dysfunction at birth had fetal thyroid measurement within the normal range. KEY WORDS: Fetal thyroid, Ultrasound, Goiter, Thyroid dysfunction.

compared these values with those obtained from a group of fetuses who were at increased risk for thyroid dysfun ction.

MATERIALS AND METHODS Measurements of the thyroid were obtained in 31 fetuses with no known risk factors for perinatal thyroid dysfunction. These fetuses had accurate gestational dating, as each one had had a first trimester sonogram that was in agreement with the menstrual age. All 31 scans were done for low-risk indications. Patients with twin gestations and patients with known ~ maternal disease, such as hypertension, diabetes, and lupus erythematosus, were excluded from the study. The measurements of the fetal thyroid were obtained using an axial scan plane through the neck at the midlevel of the thyroid. Calipers were placed on the outer dimensions of the fetal thyroid just inside the carotid vessels. An ellipse was formed using electronic calipers to encompass the body of the thyroid in transverse section (Fig. 1A). The widest diameter and circumference were recorded. These scans were all performed using an Acuson 128, variable focus, 3.5 mgHz sector transducer (Acuson, Mountain View, CA).

1992 by the American Institute of Ultrasound in Medicine • J Ultrasound Med 11:25- 28, 1992 • 0278-4297/ 92/ $3.5 0

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J Ultrasound Med 11:25- 28, 1992

THE FETAL THYROID

Twenty-three patients were referred for sonography of the fetal thyroid because of maternal thyroid disease. Sixteen patien ts were known to have Graves' disease, six patients had hypothyroidism, and one had Hashimoto's thyroiditis. An adequate axial image of the thyroid in the transverse plane was available in all cases, and these were measured retrospectively. The width was determined using calipers and the circumference calculated by using the formula for an ellipse. Neonatal follow -up of these cases for the presence or absence of thyroid disease was undertaken by interviews with the parents or pediatricians as well as review of the pediatric record . The diameters and circumferences of the thyroid in fetuses at increased risk for perinatal thyroid dysfunction were compared to those of the control group. Statistical analysis was performed on fetuses at low risk for thyroid disease, assuming a simple linear relation between thyroid width or circumference and gestational age. Linear regression was done according to accepted literature standards.4 Because we were interested in deviations greater than average, a 95% upper confidence limit for thyroid dimensions at any partie· ular gestational age was constructed on the basis of the slope of the linear regression equation for normal subjects.

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RESULTS The range of gestational ages studied for the thyroid group was between 23 and 40 weeks of fetal develop· ment; the range for the control group was between 20.5 and 41 weeks. The mean gestational age fo r each group was 31 weeks. The 31 control fetuses had thyroid widths between 11 and 29 mm and thyroid circumferences between 41 and 75 mm. An increase in thyroid size occurred with advancing gestational age (Figs. 2 and 3). For thyroid width, the regression equation was y = 10.2 ± 0,36 x where y represents the fetal thyroid width and x .represents the gestational age. For thyroid circumference, the regression equation was y ""' 28.9 ± 0.94 x, where y represents the fetal thyroid circumference and x represents the gestational age. The diameter of the thyroid for those at risk for perinatal thyroid dysfunction ranged between 12 and 42 mm, whereas the circumference ranged between 47 and 112 mm. All but five fetuses in this category had thyroid sizes within the same range as the control group and had no evidence of thyroid disease or goiter at birth, These five fetuses had sonographically demonstrable enlargement of the thyroid on six scans (one fetus was scanned twice); their thyroid widths ranged between 28 and 42 mm and thyroid circumfer· ences between 76 and 112 mm (Fig. 1B). All five fetuses with sonographkally evident enlarged thyroids in

8 Figure 1 A, Axial scan of the fetal thyroid in a patient without thyroid disease. The transverse limits of the thyroid are shown with calipers, and the diameter and circumference measurements are displayed. B, Axia·l scan of the fetal thyroid (arrows) demonstrating a goiter. The neck vessels are indicated by open arrows. The trachea can be seen between the Jobes of the thyroid.

utero were later confirmed to have neonatal thyroid dysfunction at birth. Two of these neonates had elevated thyroid stimulating hormone (TSH) levels, con· firming hypothyroidism, and three had elevated thyroxine levels, confirming neonatal hyperthyroidism. Three of these five fetuses had evidence of either

) Ultrasound Med 11:25- 28, 1992

BROMLEY ET AL

intrauterine growth retardation, oligohydramnios, or tachycardia (Table 1). Figures 2 and 3 show the measurements of the fetal thyroid widths and circumferences by gestational age in patients with and without any history or evidence of thyroid disease. The slope of the line in each graph represents the upper limit of the 95% confidence inter· val for thyroid measurements in patients without any evidence of thyroid dysfunction. The six scans performed on the five fetuses who were shown to have neonatal thyroid disorders clearly had thyroid widths

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The fetal thyroid: normal and abnormal sonographic measurements.

The thyroid of 31 fetuses at low risk for perinatal thyroid disease were evaluated sonographically. The transverse width and circumference of the feta...
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