The interaction between levothyroxine and food

Levothyroxine is the main drug used to treat primary hypothyroidism, and the third most popular drug in the US. Clinically significant interactions between this drug and food, beverages, and supplements can change the safety and efficacy of thyroid-hormone replacement therapies.

Because of the widespread use of levothyroxine, corresponding food interactions should be noted by patients, physicians, and other healthcare professionals.

Troubling behaviors

Authors of a review article published in Pharmaceuticals cited specific data from a study involving 925 hypothyroid patients that highlighted “disturbing” lifestyle and diet patterns.^[]

• 51.8% of respondents reported using dietary supplements that interact with levothyroxine (eg, calcium or iron), and 68% noted intake of food/beverages rich in fiber iodine, or soy.

• 20% of respondents took levothyroxine with breakfast, lunch, or dinner.

• 21.5% of respondents took the drug fewer than 30 minutes before meals.

Overall, 13.4% of patients complained of not being able to control their hypothyroid symptoms.

Goitrogenic foods

These effects are exacerbated by eating a lot of salt, which leads to thyroid swelling.

Examples of goitrogenic foods include cruciferous vegetables (eg, Brussels sprouts, cauliflower, cabbage, kale), nuts (eg, almonds, peanuts), sweet corn, and sorghum.

According to Thyroid UK, cooking these foods thoroughly could denature their goitrogenic enzymes, and therefore may be a good practice for those taking levothyroxine.^[]

Impact of other foods

“Soybeans and coffee have the greatest impact on the reduction of absorption, whereas vitamin C has the ability to increase it,” wrote the authors of a review published in Critical Therapeutics. “Conversely, the effect of dietary fiber on the absorption of LT4 is not yet fully understood; further research is needed on this topic.”^[]

Thyroid UK notes there’s debate over whether soya impairs levothyroxine absorption, with some studies exhibiting no effects and others exhibiting sexual-dimorphic effects in women.

Consequently, it may be a good idea to leave a 4-hour gap between soya intake and levothyroxine ingestion. Similarly, fibrous food should be avoided around the time thyroid medications are taken because they can interfere with levothyroxine absorption.

Kelp is high in iodine, and increased iodine levels impact thyroid function. Although iodine’s needed in the production of thyroid hormone, if levels are too high, thyroid overactivity—as well as paradoxical hypothyroid effects—can result.

Instead, coffee should be sipped an hour after taking levothyroxine.

Authors of the Pharmaceuticals review found that levothyroxine efficacy decreased when taken with milk, juices, or papaya.

Dosing levothyroxine

According to authors of the Pharmaceuticals review, levothyroxine tablets should be taken 60 minutes before a meal, while oral liquid and soft-gel capsules can be taken with meals to boost patient adherence. Moreover, making the switch from tablets to novel formulations can limit issues with levothyroxine-induced malabsorption secondary to coffee, calcium, or iron intake.

Also, physicians should consider consulting with a registered dietitian to help these patients receive the best possible care.