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Zyrtec and Nasonex do not interact with Synthroid directly. Although there is a very slight chance that Nasonex can increase the effect of Synthroid, this is exceedingly rare. Regardless, Synthroid should be taken on an empty stomach for the best absorption. If you are over-replaced on Synthroid and are running "hyper" (meaning that you have symptoms of hyperthyroidism from taking too much medication) there is a possibility that these medications may exacerbate your symptoms.
Medically reviewed by John A. Daller, MD; American Board of Surgery with subspecialty certification in surgical critical care
FDA Prescribing Information for Synthroid