The Grapefruit Diet and Thyroid Disease

Using this weight loss approach can pose dangerous drug interactions

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Weight gain is a common concern among people with hypothyroidism, and the grapefruit diet—a weight loss plan that began in the 1930s as the so-called "Hollywood Diet"—is one approach embraced by many. However, grapefruit can interact with thyroid replacement medication, making it hard to adjust the right dose.

A grapefruit cut in half
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About the Grapefruit Diet

The Grapefruit Diet has gone through phases of being a popular "quick-fix" diet. While its use has been traced back to Hollywood's golden age, the diet has gained renewed popularity over the years.

There are many variations of the diet, all ultimately based on the belief that grapefruit contains "fat-burning enzymes" or "speeds up" the body's natural metabolism. These claims haven't been proven, but consuming high amounts of grapefruit or grapefruit juice has been linked to weight loss.

In the 1970s and early 1980s, grapefruit was embraced anew in "10-day, 10-pounds-off" diets for which the fruit was to be eaten with every meal. This diet was usually promoted without attention to exercise or any other dietary intervention.

Despite claims that grapefruit was a natural "fat-burner," the weight loss was achieved almost entirely by the intake of fewer calories rather than any specific property related to grapefruit.

Again, in the early 2000s, a renewed interest in grapefruit in maintaining glycemic control in people with insulin resistance led to findings that once again placed the fruit in the weight loss spotlight.

In 2006, endocrinologist Ken Fujioka and his colleagues at the Scripps Clinic published a study in which obese people who drank a 7-ounce glass of grapefruit juice three times a day for 12 weeks lost an average of 1.6 kilograms (3.52 pounds), while those who drank 7-ounce of apple juice only lost 0.3 kilograms (0.66 pounds). Some of the participants were reported to have lost as much as 10 kilograms (about 22 pounds).

Despite the fact that the study was small (only 91 participants) and the researchers could offer no scientific explanation for the weight loss, the results were enough to place grapefruit in many diet plans.

The problem is that diet plans are never as simple as proponents claim. And if you take thyroid hormone replacement therapy, grapefruit may cause more harm than good.

Thyroid Drug Interactions

Grapefruit can be a delicious and nutritious addition to a diet or meal plan. It is the third most commonly consumed citrus fruit in the United States, behind oranges and lemons, and offers a glycemic index of 6 (meaning that it is less likely to affect your insulin levels). Moreover, it is rich in vitamin C, vitamin A, folic acid, potassium, fiber, and flavonoids.

But grapefruit poses a potential risk for people who take thyroid hormone replacement therapy.

Grapefruit interferes with an enzyme known as CYP3A4, which the intestines use to break down certain compounds so that they can be absorbed. Repeated doses of grapefruit juice may also inhibit the activity of hepatic (liver) CYP3A4. Many thyroid drugs, including levothyroxine, rely on CYP3A4 for metabolization and absorption.

With grapefruit consumption, the bioavailability of thyroid medication (the amount that enters the bloodstream) can be significantly reduced, reducing the benefits of therapy.

While the occasional grapefruit or glass of juice will likely do you little harm, routine or excessive consumption may be problematic. A case study published in the British Journal of Clinical Pharmacology detailed an incidence in which the heavy consumption of grapefruit in a 36-year-old woman on levothyroxine reduced her thyroid hormone levels below therapeutic values. A simple reduction in grapefruit intake normalized the values.

Other Medications Affected By Grapefruit Juice

Thyroid drugs are not the only agents affected by grapefruit. Grapefruit also increases the plasma concentrations of other drugs that are substrates for CYP3A4, for example, felodipine, cyclosporin, and simvastatin.

And recently, grapefruit juice has been reported to decrease the plasma concentrations of some drugs, for example, fexofenadine, levothyroxine, and celiprolol—possibly by inhibiting intestinal uptake transporters, rather than through CYP3A4 inhibition.

Some drugs affected by grapefruit include:

  • Some anti-anxiety drugs like buspirone
  • Some anti-arrhythmia drugs like Pacerone (amiodarone)
  • Some antihistamines like Allegra (fexofenadine)
  • Some corticosteroids like Entocort EC (budesonide)
  • Some high blood pressure medications like Procardia (nifedipine)
  • Some statin drugs like Zocor (simvastatin) and Lipitor (atorvastatin)
  • Some organ-transplant rejection drugs like Sandimmune (cyclosporine)

Consuming Grapefruit Safely

The main challenge of a grapefruit diet is that it requires you to consume significant quantities of grapefruit on an ongoing basis. While there is no set amount that is considered safe or unsafe, the British researchers concluded that consuming 7 ounces of grapefruit juice three times daily for only two days translated to a 10% decline in the absorption of levothyroxine.

With that being said, other variables, such as weight and the severity of thyroid disease, may also contribute to the risk.

It is important to speak with your healthcare provider to discuss whether grapefruit is appropriate for you. In most cases, your healthcare provider will advise you to avoid overconsumption, to separate your levothyroxine dose and grapefruit intake by four hours, and to check your thyroid levels routinely to make sure they remain in a safe range. In addition, if your weight fluctuates by more than 10 to 15 pounds, your thyroid dose may need to be adjusted.

A Word From Verywell

Insofar as weight loss is concerned, there are many options to consider. While grapefruit may seem like an attractive way to lose weight quickly, it is important to remember that there is no such thing as a quick fix. A healthy, balanced diet that meets your nutritional needs is the best option.

If you're struggling with your weight, ask your healthcare provider for a referral to a nutritionist experienced in thyroid disease who can discuss the various options and help you embark on a sustainable exercise program tailored to your fitness level. This, along with optimal thyroid drug adherence, will put you on the road to gradual and sustainable weight loss.

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4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Fujioka K, Greenway F, Sheard J, et al. The effects of grapefruit on weight and insulin resistance: Relationship to the metabolic syndrome. J Medicinal Food. 2006;9(1); published ahead of print. doi:10.1089/jmf.2006.9.49

  2. Tanguay M, Girard J, Scarsi C, Mautone G, Larouche R. Pharmacokinetics and comparative bioavailability of a levothyroxine sodium oral solution and soft capsuleClin Pharmacol Drug Dev. 2019;8(4):521–528. doi:10.1002/cpdd.608

  3. Lilja J, Laitinen K, Neuvonen P. Effects of grapefruit juice on the absorption of levothyroxine. Bri J Clin Pharmacol. 2005;60(3):337-41. doi:10.1111/j.1365-2125.2005.02433.x

  4. Bailey DG, Dresser G, Arnold JM. Grapefruit-medication interactions: forbidden fruit or avoidable consequences?CMAJ. 2013;185(4):309–316. doi:10.1503/cmaj.120951

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