From the FDA Drug Label
Evaluate the need for dosage adjustments when regularly administering within one hour of certain foods that may affect levothyroxine sodium tablets absorption Do not administer in foods that decrease absorption of levothyroxine sodium tablets, such as soybean-based infant formula Soybean flour, cottonseed meal, walnuts, and dietary fiber may bind and decrease the absorption of levothyroxine sodium from the gastrointestinal tract.
Consuming soy within 30 minutes of taking levothyroxine (Synthroid) may be significant, as it may decrease the absorption of levothyroxine sodium from the gastrointestinal tract.
- Key points:
- Certain foods, including soy, may affect levothyroxine sodium absorption.
- Dosage adjustments may be necessary when administering levothyroxine sodium with these foods.
- It is recommended to administer levothyroxine sodium on an empty stomach, one-half to one hour before breakfast, to minimize potential interactions with food. 1 1 1
From the Research
Consuming soy within 30 minutes of taking levothyroxine (Synthroid) is significant and should be avoided due to its potential to interfere with thyroid hormone absorption, which can lead to inadequate treatment of hypothyroidism and impact morbidity, mortality, and quality of life. The most recent and highest quality study, 2, published in 2021, highlights the importance of considering the impact of dietary supplements, including those containing soy, on levothyroxine absorption. This study demonstrates that over-the-counter protein supplements, which may contain soy lecithin, can impair thyroxine absorption in hypothyroid patients.
Key Considerations for Levothyroxine Absorption
- Levothyroxine is best absorbed when taken on an empty stomach, typically 30-60 minutes before breakfast, with a full glass of water.
- Soy contains compounds called isoflavones that can interfere with the absorption of levothyroxine in the digestive tract, potentially reducing the amount of medication that enters the bloodstream.
- Other foods and supplements that can affect levothyroxine absorption include calcium-rich foods, iron supplements, antacids containing aluminum or magnesium, and high-fiber foods.
Recommendations for Patients Taking Levothyroxine
- Wait at least 4 hours after taking levothyroxine before consuming soy products or other substances that may interfere with absorption.
- Establish a consistent daily routine for taking levothyroxine at the same time each day, separate from meals and other medications or supplements.
- Be cautious when taking over-the-counter protein supplements, as they may contain additives like soy lecithin that can interfere with levothyroxine absorption, as noted in 2.
- Monitor thyroid-stimulating hormone (TSH) levels regularly to ensure that levothyroxine doses are adequate and adjust as necessary, considering the potential impact of dietary factors as discussed in 3, 4, 5, and 6.