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Newborns and very young children should undergo thyroid monitoring in the weeks after undergoing certain medical imaging, the .

In an update to a , the agency is newly recommending that newborns and children up to 3 years old undergo follow-up thyroid monitoring within 3 weeks after receiving injections of contrast media containing iodine -- so-called "contrast dye" or "x-ray dye."

This new warning pertains to the entire class of iodinated contrast media (ICM) injections, which are commonly used for x-rays of blood vessels, joints, and organs, and some CT scans.

Following a review of new research, the FDA determined that these injections have the potential to lead to an underactive thyroid or a temporary decrease in thyroid hormone levels. Although this was a relatively uncommon occurrence, the agency recommended these possible thyroid conditions should be diagnosed and treated as early as possible to prevent future issues.

This is particularly true for the most vulnerable population in question: premature newborns, very low birth weight newborns, or children with underlying conditions such as cardiac issues that could raise the risk for other thyroid issues.

"If thyroid dysfunction is detected, treat and monitor thyroid function as clinically needed to avoid future cognitive and other developmental disabilities," the FDA said in its recommendation to healthcare providers, suggesting these patients may require transient T4 replacement therapy.

The agency also advised parents and caregivers to keep in mind that newborns and young children often won't show any visible signs of thyroid issues.

Underpinning this new warning, the FDA reviewed six studies that have been published after and five studies published prior to their initial 2015 alert.

In total, their review included data on 3,481 patients under 3 years of age who were exposed to ICM. Across these studies, thyroid dysfunction was reported in anywhere from 1% to 15% of the patients, and was most common in preterm neonates. Most of these cases were transient subclinical hypothyroidism and didn't require treatment. Those at high risk were patients with underlying cardiac issues since they typically require higher doses of contrast during CT imaging for catheterization.

While most thyroid dysfunction occurred within 3 weeks of exposure, diagnosis in the literature review ranged from 8.5 to 138 days.

These injections -- currently there are six approved, including iodixanol (Visipaque), iohexol (Omnipaque), iopamidol (Isovue), iopromide (Ultravist), iothalamate meglumine (Conray), and ioversol (Optiray) -- have been around for several decades and are commonly administered into arteries, veins, or other body cavities. However, this particular safety warning only pertains to ICM injections given into artery or veins.

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