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. Unfortunately, any benefits of a grapefruit-based diet may end up causing you more harm than good.

A grapefruit cut in half
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About the Grapefruit Diet

The Grapefruit Diet is one of the most popular "quick-fix" diets. While its use has been traced back to Hollywood's golden age, the diet has gained renewed popularity in recent years. While there are many variations of the diet, they are all ultimately based on the belief that grapefruit contains "fat-burning enzymes" or somehow "speeds up" the body's natural metabolism.

In the 1970s and early 1980s, grapefruit was embraced anew in so-called "10-day, 10-pounds-off" diets for which the fruit was to be eaten with every meal (and usually without the need for 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.

However, 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 thrice daily for 12 weeks lost an average of 1.6 kilograms (3.52 pounds) compared to those drink 7-ounce of apple juice who 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 solidify grapefruit as a staple food of many diet plans.

The problem, of course, is that diet plans are never as simple as proponents claim. This is especially true with people on thyroid hormone replacement therapy for whom grapefruit may cause more harm than good.

Thyroid Drug Interactions

There's no doubt that grapefruit is a delicious and nutritious addition to any diet. 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 to people on thyroid hormone replacement therapy. This is because grapefruit interferes with an enzyme known as CYP3A4 which the intestines use to break down certain compounds so that they can be absorbed. Many thyroid drugs, including levothyroxine, rely on CYP3A4 for metabolization and absorption.

If CYP3A4 is inhibited due to grapefruit consumption, the bioavailability of a thyroid drug (the amount that enters the bloodstream) can be significantly reduced, undermining 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.

Thyroid drugs are not the only agents affected by grapefruit. Others 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 not that it may not work as well as its proponents claim; it's 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, they also insisted that the slowed absorption of levothyroxine due to grapefruit consumption doesn't inherently affect the bioavailability of the drug. Other variables, such as weight and the severity of the hypothyroid 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 and separate your levothyroxine dose and grapefruit intake by four hours and 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 other options beyond the grapefruit diet 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 a quick fix. A healthy, balanced diet that meets your calorie needs is the best option.

If 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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