Transient Hyperintensity of the Infant Thyroid Gland on T1-Weighted MR Imaging: Correlation with Postnatal Age,Gestational Age,and Signal Intensity of the Pituitary Gland

BACKGROUND AND PURPOSE:The signal intensity of the thyroid in neonates is high on T1WI. It is affected by gestational and postnatal ages. However, the extent of the influence of these ages is unknown. This study investigated the relationship of signal intensities of the infant thyroid with postnatal and gestational ages and anterior pituitary using 3D gradient-echo T1WI.MATERIALS AND METHODS:This retrospective study included 183 T1-weighted images from 181 infants. Using a multiple linear regression analysis, we evaluated the effects of postnatal and gestational ages on the thyroid–muscle signal intensity ratio. The relationship between the thyroid and anterior pituitary signal intensities on T1WI and the age of the infants was evaluated.RESULTS:Multiple linear regression analysis showed that the thyroid signal intensity was affected negatively by postnatal age at examination and positively by gestational age at birth (P < .01 and P = .04, respectively). According to the standardized partial regression coefficients, the influence of postnatal age at examination was stronger than that of gestational age at birth (−0.72 and 0.13, respectively). The thyroid and anterior pituitary signal intensities reached constant values at 12 weeks'' postnatal age, and the mean thyroid–anterior pituitary signal intensity ratios were almost 1 throughout the entire period.CONCLUSIONS:The signal intensity of the infant thyroid on T1WI was more strongly influenced by the postnatal age at examination than the gestational age at birth, and it was almost equal to that of the anterior pituitary.

Thyroid hormone is essential for prenatal and neonatal neurologic development.¹ The fetal hypothalamic–pituitary–thyroid axis begins to develop by gestational week 6, and thyroid follicular epithelial cells are able to concentrate iodide and synthesize thyroid hormone by 11 weeks of gestation.^2,3 After delivery, serum thyroid-stimulating hormone (TSH) demonstrates a transient increase in the first 24 hours, and this TSH surge stimulates a rise in serum levels of thyroxine and triiodothyronine.⁴ A previous study reported that the fetal thyroid gland showed high signal intensity on 3D gradient-echo (GRE) T1WI, and the fetal thyroid–muscle signal intensity ratio did not correlate with gestational age.⁵In our institution, infants in the neonatal intensive care unit are routinely screened for CNS abnormalities using MR imaging, including 3D GRE T1WI in the head to neck range. Some of these infants display high signal intensity in the thyroid gland on 3D GRE T1WI, especially when the time between birth and the MR examination was short. The anterior pituitary glands of neonates usually show high signal intensity on T1WI, and the relative signal intensity of the anterior pituitary gland to that of the pons was significantly negatively correlated with postnatal age at examination.⁶ A recent study reported that the signal intensity of the infant thyroid and anterior pituitary on 3D GRE T1WI correlated positively with the gestational age at birth and negatively with the postnatal age at examination.⁷ However, the extent of the influence of gestational age at birth and postnatal age at examination on the signal intensities of the infant thyroid and anterior pituitary on T1WI is not known.It was hypothesized that postnatal age at examination might affect the signal intensity changes of the infant thyroid on 3D GRE T1WI more strongly than the gestational age at birth. The change in signal intensity of the infant thyroid on 3D GRE T1WI was thought to be similar to that of the anterior pituitary. In this study, a multiple linear regression analysis was used to evaluate the effects of postnatal age at examination and gestational age at birth on the thyroid–muscle and anterior pituitary–muscle signal intensity ratios. In addition, the change in signal intensity of the infant thyroid gland on 3D GRE T1WI was assessed to clarify the relationship between signal intensities of the infant thyroid and anterior pituitary.