Synthroid and Imaging Contrast Dye: What You Need to Know Before Your Scan

At a glance

• Drug / Levothyroxine sodium (Synthroid), oral T4 replacement [1]
• Label cited / Generic levothyroxine sodium tablets label, not the Synthroid-brand label [1]
• Contrast type / Iodinated contrast media used for CT and angiography
• Label Table 5 / Does not name iodinated contrast media [1]
• Label spacing rule / At least 4 hours before or after drugs known to interfere with absorption [1]
• Label absorption list / Calcium carbonate, ferrous sulfate, sevelamer, lanthanum, orlistat, bile acid sequestrants, ion exchange resins, PPIs, sucralfate, antacids [1]
• Dose change before scan / No contrast-specific dose instruction in Table 5 or the administration section [1]

Is there a known interaction?

The levothyroxine sodium tablets label's Table 5, which lists drugs that may decrease T4 absorption, does not name iodinated contrast media [1].

What does the label warn about instead?

Table 5 covers drugs that can bind levothyroxine or change gut acidity. It names calcium carbonate, ferrous sulfate, sevelamer, lanthanum, orlistat, bile acid sequestrants, ion exchange resins, proton pump inhibitors, sucralfate and antacids [1]. For calcium carbonate and ferrous sulfate, the label says to administer levothyroxine at least 4 hours apart [1]. For bile acid sequestrants and ion exchange resins, it says to give levothyroxine at least 4 hours before them or monitor TSH [1]. For PPIs, sucralfate and antacids, it says to monitor patients and gives no fixed hour spacing [1]. None of these instructions is written for iodinated contrast media [1].

Does the label say to change the dose before a scan?

Neither Table 5 nor the administration section gives a contrast-specific instruction to change the levothyroxine dose before or after iodinated contrast [1]. The administration section says to take levothyroxine once daily on an empty stomach, one-half to one hour before breakfast, and at least 4 hours before or after drugs known to interfere with absorption [1]. The label also says levothyroxine has a narrow therapeutic index and that both overtreatment and undertreatment can have negative effects [1].

Is a TSH check after contrast covered by the label?

The label sections checked do not set a TSH-check interval for iodinated contrast [1]. The label says adequacy of levothyroxine therapy is determined with periodic TSH and/or T4 monitoring and clinical status [1]. That is its general monitoring statement, not a post-contrast schedule.

Frequently asked questions

›Does iodinated contrast dye interact with levothyroxine?

The levothyroxine sodium tablets label's Table 5 does not name iodinated contrast media [1]. Its absorption table names calcium carbonate, ferrous sulfate, sevelamer, lanthanum, orlistat, bile acid sequestrants, ion exchange resins, proton pump inhibitors, sucralfate and antacids [1].

›Does the label say to stop levothyroxine before a contrast scan?

No. The label gives no contrast-specific instruction to stop levothyroxine or change the dose [1]. It does say that undertreatment and overtreatment both carry risks because levothyroxine has a narrow therapeutic index [1].

›Does the label require a TSH check after contrast?

The label sections checked set no TSH-check interval for iodinated contrast. The label says adequacy of levothyroxine therapy is determined with periodic TSH and/or T4 monitoring and clinical status [1].

›What is the 4-hour levothyroxine rule?

The label says to administer levothyroxine sodium tablets at least 4 hours before or after drugs known to interfere with absorption [1]. It names calcium carbonate and ferrous sulfate as agents to separate by at least 4 hours, and bile acid sequestrants and ion exchange resins as agents to give levothyroxine at least 4 hours before or monitor TSH [1].

References

1. U.S. National Library of Medicine, DailyMed. Levothyroxine sodium prescribing information. Levothyroxine sodium label