Drug Interactions Newsletter

Volume 1, Issue 33, 09/01/2009

Question

What is the specific mechanism by which aluminum hydroxide inhibits the absorption of levothyroxine (T4; Levothroid®, Levoxyl®, Synthroid®, and Unithroid®)?

Answer

The oral antacid, aluminum hydroxide, is commonly used for the treatment of acid indigestion, gastroesophageal reflux disease and even to reduce phosphate absorptions in patients with hyperphosphatemia secondary to chronic kidney disease.  Due to its presence as an ingredient in several over-the-counter (OTC) products, such as Maalox® and Mylanta®, it is plausible that it could be used by patients without the consultation or knowledge of a healthcare provider.  While this may not be a problem if that patient is on no other medications, it could result in the loss of thyroid control in hypothyroid patients taking levothyroxine (synthetic T4) therapy.^1-3  This would be especially true in patients who take aluminum containing products within 2-4 hours of taking their levothyroxine.^1,3  In fact, pharmacokinetic drug interaction studies have shown that thyroid stimulating hormone (TSH; thyrotropin) concentrations increase from an average of 2.62 mU/L up to 7.19 mU/L (normal range or control 0.5 - 5.0 mU/L).³  This increase would suggest a significant loss of thyroid control.  It appears that the interaction comes from a reduction in the absorption of levothyroxine.  Most clinicians will say that aluminum or other di- or tri-valent cations will "chelate" with levothyroxine to prevent absorption, but what does that actually mean?

......To read the full answer please LOGIN or SUBSCRIBE NOW.