From the Research
It's generally not safe to consume soy milk 30 minutes after taking levothyroxine, and you should wait at least 4 hours after taking your thyroid medication before consuming soy products, including soy milk. Levothyroxine is best absorbed on an empty stomach, and soy contains compounds that can interfere with the absorption of the medication, potentially reducing its effectiveness, as shown in a study published in Pediatrics 1. This interference occurs because soy contains phytoestrogens and proteins that can bind to the medication in your digestive tract. For optimal results, take levothyroxine first thing in the morning with a full glass of water, at least 30-60 minutes before eating or drinking anything else (except water), and wait the full 4 hours before consuming soy products. Some studies have investigated the effect of other substances on levothyroxine absorption, such as milk 2 and tea 3, but the most relevant study to this question is the one focusing on soy products 1. If you regularly consume soy products, try to do so consistently at the same time each day to maintain stable thyroid hormone levels. If you accidentally consume soy too soon after taking levothyroxine, don't take an extra dose, but try to be more careful with timing in the future. It's also worth noting that temporal separation of the intake of soy protein from the administration of levothyroxine can result in attainment of suppressive serum levels of free T4 and TSH with use of lower doses of levothyroxine, as reported in a case study published in Endocrine Practice 4. However, the most recent and highest quality study on this topic is not available, but based on the available evidence, it is recommended to wait at least 4 hours after taking levothyroxine before consuming soy products.