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The Effects of Grapefruit on Synthroid

author image Kathryn Gilhuly
Kathryn Gilhuly is a wellness coach based in San Diego. She helps doctors, nurses and other professionals implement lifestyle changes that focus on a healthy diet and exercise. Gilhuly holds a Master of Science in health, nutrition and exercise from North Dakota State University.
The Effects of Grapefruit on Synthroid
Synthroid is not known to be negatively affected by grapefruit. Photo Credit Purestock/Purestock/Getty Images

Grapefruit adversely reacts with many prescription drugs. It can elevate the level of medication in your blood, increasing your risk for serious side effects. However, Synthroid, a brand-name thyroid replacement medication, does not interact dangerously with grapefruit, based on evidence available as of 2011. But new medical and drug interaction information could change this assessment. If you take Synthroid or any other medication, ask your doctor or pharmacist about updates on their interactions with grapefruit.

No Known Interaction

MedlinePlus, a service of the National Institutes of Health, does not list grapefruit among its interaction warnings for Synthroid. Additionally, the United States Food and Drug Administration’s most currently available label for Synthroid, approved on July 14, 2008, does not list grapefruit as a substance to avoid while taking Synthroid. According to the People’s Pharmacy, however, the majority of drugs remain untested for possible interactions with grapefruit.

CYP34A Enzyme Inhibition

Grapefruit reacts adversely with many medications because it inhibits your CYP34A enzyme from metabolizing drugs. Some drugs and some people are more sensitive to grapefruit's effect on this enzyme. If grapefruit slows the metabolism of a drug you take, you may end up with an excess of the medication in your system -- an overdose. An elevated dose of medication may increase the effectiveness of a medication and your risk for side effects.

Synthroid Uses and Side Effects

Synthroid treats a condition called hypothyroidism. Hypothyroidism develops when your body does not naturally produce enough of the thyroid hormone. Doctors may also prescribe Synthroid to treat goiter, a condition caused by an enlarged thyroid gland. Side effects of Synthroid include vomiting, nausea, tremors, weight loss, insomnia and hair loss. Theoretically, combining grapefruit with Synthroid could increase your risk for these symptoms. However, no evidence available as of 2011 suggests that it will.

Known Drug Interactions

Drugs known to interact adversely with grapefruit include oral contraceptives, statins, calcium channel blockers, antidepressants, anti-anxiety medications and immunosuppressants. This is simply a partial list. For a full list of known drug interactions with grapefruit, consult a medical professional. Serious side effects include blood clots, gangrene, strokes and heart attacks .A single glass of grapefruit may slow your body's metabolism of a drug by 47 percent, according to the Harvard Medical School Family Health Guide.

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