Q. I'm taking 0.625/2.5 mg tablet of Prempro daily. I also take a daily dosage of levoxyl 25 mcg. I have read that estrogen drugs interfere with the effectiveness of thyroid drugs. Does the Prempro diminish the effectiveness of the Levoxyl?

A. It is well known that many drugs can interact, resulting in an increase or decrease in needed dosing. The most common cause of this has to do with drug effects on the liver, where most medications are metabolized (broken down into smaller molecules). One drug can block the enzyme that breaks down a second drug, making that second drug more potent for example. There are published studies that women taking estrogen may need a higher dose of thyroid hormone. Periodic blood tests ordered by your doctor can establish the correct dose of thyroid medication in this situation.

http://www.nlm.nih.gov/medlineplus/druginfo/medmaster/a601041.html

http://www.nlm.nih.gov/medlineplus/druginfo/uspdi/202566.html

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