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Original Research |
1 Department of Radiology, Mayo Clinic, 200 First St. SW, Rochester, MN
55902.
2 Department of Pediatric Endocrinology and Metabolism, Mayo Clinic, Rochester,
MN.
3 Public Health Laboratory, Minnesota Department of Health, St. Paul, MN.
Received October 23, 2007;
accepted after revision January 14, 2008.
Address correspondence to T. D. Atwell
(atwell.thomas{at}mayo.edu).
Abstract
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MATERIALS AND METHODS. We identified pregnant patients ("maternal patient") who underwent CT with iodinated IV contrast material between February 2000 and October 2006. The specifics of the CT examinations were reviewed including patient age, gestational age of conceptus at time of exposure, type of CT, and type and dose of contrast agent. The neonatal serum thyroid-stimulating hormone (TSH) level obtained from the exposed infant ("neonatal patient") was retrieved from the Minnesota Department of Health.
RESULTS. Twenty-one patients gave us consent to review the requested data concerning their 23 children (two mothers had twins). Mean maternal age at the time of CT was 29 years (range, 19–41 years). Mean gestational age (based on last menstrual period) at the time of CT was 23 weeks (range, 8–37 weeks). Neonatal patients were born at a mean of 38 weeks of gestation (range, 24–41 weeks of gestation). Serum TSH was collected at a mean of 43 hours after delivery (range, 24–70 hours). For all neonatal patients, serum TSH levels were normal. Mean serum TSH was 9.7 µIU/mL (range, 2.2–28.8 µIU/mL). No maternal patient reported thyroid trouble in her child.
CONCLUSION. Based on neonatal TSH measurements in a small number of patients, we found no ill effect of iodinated contrast agents on neonatal thyroid function after in utero exposure.
Keywords: contrast media • fetal imaging • hypothyroidism • neonatal imaging • obstetrics • thyroid function • thyroid-stimulating hormone
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Imaging a pregnant patient using CT challenges the radiologist to manage both the radiation risk to the fetus and the questionable effect of iodinated contrast agents on fetal thyroid function. Investigators of previous studies in the literature have stated that administration of medications containing iodine to the pregnant patient can result in goiter formation and thyroid dysfunction in the neonate and infant, although the true incidence of hypothyroidism has not been accurately assessed [2]. Little experience has been published regarding the effects of the currently available iodinated nonionic water-soluble contrast media on the neonatal thyroid.
The Contrast Media Safety Committee of the European Society of Urogenital Radiology recently released guidelines regarding the use of contrast agents during pregnancy [3]. They specifically stated the following:
In view of the paucity of information in the literature, collection and publication of neonatal thyroid function results after maternal exposure to iodinated contrast media would be most helpful.
Currently, the Minnesota Department of Health (MDH) screens all newborns for congenital hypothyroidism by measuring thyroid-stimulating hormone (TSH) levels. In the setting of hypothyroidism, this hormone will be abnormally elevated. Coordinating with the MDH, we documented neonatal thyroid function after in utero exposure to iodinated contrast material.
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An IRB-approved consent form was sent to 51 maternal patients who fit our inclusion criteria. The form explained the purpose of the study and which contrast-enhanced CT examination had been performed, including the date of the CT examina tion. We requested written permission to obtain the result of their child's (neonatal patient's) neonatal serum TSH test obtained during the neonatal period by state mandate and held by the MDH. Each mother also was asked to provide her child's birth date and to describe any history of thyroid illness to the best of her ability. A second letter was sent to the patients who did not initially respond.
Gestational age of the neonate at delivery was determined by correlating existing information in the medical record specifying gestational age at the time of CT with the actual date of birth.
The specifics of the maternal CT examinations were reviewed including maternal age and gestational age at the time of CT, body part imaged, and type and dose of IV contrast material.
Minnesota state law (statute 144.125) requires that hospitals and care providers arrange blood spot testing for all newborns unless a parent objects in writing. Dried blood spots are screened for congenital hypothyroidism using a fluoroimmuno assay to measure TSH level. The ranges of MDH-recorded serum TSH levels are detailed in Table 1; in April 2004 and October 2006, accepted TSH values were adjusted for neonatal age.
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The IV contrast material administered to the 21 maternal patients included iopamidol (Isovue, Bracco Diagnostics) and iohexol (Omnipaque, GE Healthcare). Specifically, Omnipaque 300 was administered to 18 patients (median dose, 100 mL; range, 80–200 mL), Omnipaque 350 to one patient (80 mL), Isovue 300 to one patient (80 mL), and Isovue 370 to one patient (100 mL).
Neonatal patients were born at a mean gestational age of 38 weeks (median, 39 weeks; range, 24–41 weeks). Serum TSH was collected at a mean of 43 hours after delivery (median, 39 hours; range, 24–70 hours).
For all neonatal patients, serum TSH levels were normal. The mean serum TSH level was 9.7 µIU/mL (median, 8.7 µIU/mL; range, 2.2–28.8 µIU/mL). One neonatal patient born at 24 weeks of gestation had the TSH level evaluated a second time using serum collected 102 hours after delivery; the second TSH level was 4.9 µIU/mL. The mean age of the neonatal patients at the time of maternal consent to this study was 33 months (SD, 19 months). At the time of consent, none of the maternal patients reported known thyroid illness in any child.
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The fetal thyroid is highly sensitive to acute iodine overload [4]. Elevated levels of plasma iodine inhibit biosynthesis and secretion of thyroid hormones, an auto regulatory process known as the Wolff-Chaikoff effect [5]. The fetal thyroid gland may not be able to escape from the transient inhibitory effect of excess iodine until approximately 36–40 weeks of gestation. Susceptibility to fetal hypothyroidism after iodine exposure is therefore present between 12 and 40 weeks of gestation but may be greatest between 18 and 36 weeks when iodine uptake is increased and the escape phenomenon from the Wolff-Chaikoff effect is not fully mature [6].
Iodine and iodinated contrast agents have been shown to cross the placenta, with opacification of the fetal gut [7–9]. It is not known if contrast agent diffuses across the placenta into the amniotic fluid and is then swallowed into the gut or if contrast agent passes through the umbilical vein into the fetal circulation.
Free iodide associated with iodinated contrast material can occur as a contaminant or as a result of deiodination [10]. Free iodide as a contaminant is known to occur in both ionic and nonionic iodinated contrast media, with one study showing no significant differ ence between the two [11]. Both iohexol and iopamidol are believed to undergo minimal deiodination after administration (Omnipaque package insert, GE Healthcare, June 2006; Isovue package insert, Bracco Diagnostics, July 2002). However, the amount of deiodi nation that occurs is dependent on circulat ing time. In the fetus, prolonged exposure to contrast agent occurs due to placental recirculation, so this deiodination may become more significant.
Maternal administration of iodide (so di um iodide) or exposure to iodine-containing medications (e.g., povidone iodine) has been shown to induce changes in neonatal thyroid function in both animals [12] and humans [13].
Additional studies in the literature regard ing humans have primarily included experi ence with lipid-soluble and hyperosmolar, water-soluble iodinated contrast material. Amniofetography using combined lipid-sol uble (io dized oil [Lipiodol, Andre Guerbet]) and water-soluble (meglumine diatrizoate) con trast agents caused impairment of thyroid function in six of seven newborns, as mea sured by serum TSH levels [14]. In contrast to that experience, the isolated use meglumine diatrizoate for amniofetography showed no ef fect on neonatal cord blood T4 and T3 levels compared with a similar-sized control group [15].
This difference may be explained by the duration of iodide exposure. In two pregnant patients who underwent excretory urography using water-soluble iothalamate, one patient was found to have a marked elevation of amniotic total iodine (380 µg/mL) 24 hours after IV contrast administration, whereas the second patient had an amniotic iodine level of 29 µg/mL 22 days after iothalamate administration; neonatal TSH levels were normal in both cases [16]. Given the apparent transient nature of excess amniotic fluid iodine after exposure to water-soluble IV contrast material, these authors concluded that lipo soluble (e.g., Lipiodol) iodinated contrast agents may have greater risk of fetal effect due to relative poor excretion, and thus prolonged exposure of the fetal thyroid to excess iodine.
Our data support the hypothesis that maternal exposure to nonionic iodinated contrast agents does not affect neonatal thyroid function, as measured by neonatal TSH levels. Given the evidence presented in the literature, we suspect that the overall amount of free excess iodide available to the fetal thyroid is small and that exposure is transient.
Our study has a few important limitations, notably a bias created by the small number of patients despite extended efforts to contact potential participants. One could consider that a parent with a child who has hypothyroidism may be reluctant to participate. In addition, because of the retrospective nature of the study, we were not able to rule out the possibility of a transient in utero effect.
In conclusion, based on neonatal TSH measurements in a small number of patients, the IV administration of water-soluble nonionic iodinated contrast agents to pregnant patients had no subsequent effect on neonatal thyroid function.
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