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Thyroid Medications Can Have Harmful Interactions With Other Drugs, Public Citizen Says

Sept. 3, 2008 

Thyroid Medications Can Have Harmful Interactions With Other Drugs, Public Citizen Says

29 Medications That May Cause Adverse Interactions Identified on WorstPills.org

WASHINGTON, D.C. – Thyroid medications, which are among the most frequently prescribed medications in America, may adversely interact with common over-the-counter and prescription drugs, Public Citizen writes in a new September posting on its WorstPills.org Web site.

The effects of these interactions can include diminished effectiveness of the thyroid medication, diminished effectiveness of the other drugs and life-threatening effects on the blood’s ability to clot. Doctors can prevent adverse outcomes by adjusting the dosage of the drugs and, for some of the interactions, educating patients about appropriate times, relative to each other, to take the drugs.

In 2007 alone, doctors prescribed the thyroid medication levothyroxine to more than 75 million Americans, making it one of the most prescribed drugs in the country. The medication (sold as Levo-T, Levoxyl, Novothyrox, Synthroid, Thyro-Tabs and Unitroid) supplements the missing or deficient thyroid hormone to prevent hypothyroidism.

Hypothyroidism is an endocrine disorder that affects about 4.6 percent of the U.S. population; symptoms include fatigue, weight gain, constipation, depression and muscle cramps. 

Among the drugs that decrease the body’s ability to absorb levothyroxine are antacids such as Tums and Rolaids, iron supplements and some medications for stomach ulcers and osteoporosis. For the most part, the decreased-absorption effect can be prevented by taking the medications two to four hours apart, said Dr. Sidney Wolfe, director of the Health Research Group at Public Citizen.

Drugs that speed up how the body metabolizes the thyroid hormone also diminish the drug’s efficacy. These include enzyme inducers used to treat epilepsy, St. John’s wort and depression medication like sertraline (Zoloft). 

Estrogens may trap the thyroid medication in the bloodstream, also decreasing its effectiveness; this means estrogen replacements and oral (and patch) contraceptives also can render the medication less effective. In such cases, a physician may need to prescribe a higher dose of levothyroxine. 

Interactions with the anti-clotting medication warfarin (Coumadin) can range from negligible to life-threatening. Depending on the patient’s thyroid condition, warfarin interacts with the thyroid medication to make the blood too thin – increasing the likelihood of bleeding – or it can thicken the blood – increasing the likelihood of dangerous clots. Physicians should perform frequent blood tests to closely monitor patients beginning a regime that includes this drug combination, Wolfe said.

He added, “Levothyroxine interactions are commonly encountered in medical practice because levothyroxine is widely prescribed, as are many other of the interacting drugs. But proper testing and dosage adjustments can prevent adverse outcomes. Those taking levothyroxine must be aware of these interactions and take remedial action before any side effects occur.”

WorstPills.org includes the full list of the 29 implicated drugs cited in the article.

Worst Pills, Best Pills is a monthly newsletter available in print and electronic formats through Public Citizen’s subscription Web site, . The article about harmful interactions with thyroid medication will be available free for the next seven days. The site has other searchable information about the uses, risks and adverse effects associated with prescription medications, including all the information contained in Public Citizen’s best-selling book, “Worst Pills, Best Pills.”

WorstPills.org is an unbiased analysis of information from a variety of sources, including well-regarded medical journals and unpublished data obtained from the Food and Drug Administration, that allows Public Citizen to sound the alarm about potentially dangerous drugs long before they are banned by the federal government. For example, Public Citizen warned consumers about the dangers of Vioxx, ephedra, Baycol and Propulsid years before they were pulled from the market.