Thyroid and GLP-1: what you need to know before combining the two

When it comes to medication, understanding how different drugs interact with each other makes all the difference in treatment outcomes. If you take levothyroxine to manage your thyroid and also need help losing weight with a GLP-1 agonist, you should know that these two treatments can intersect in ways that require attention. PeptPro helps you log every symptom and dose so that the combination works in your favor. Download here: https://apps.apple.com/us/app/peptide-tracker-peptpro/id6764484462

Semaglutide and other GLP-1 medications have become widely used in clinical practice as effective tools for obesity and glycemic control. At the same time, millions of people live with hypothyroidism and rely on levothyroxine to keep their hormones balanced.

How GLP-1 interferes with thyroid hormone absorption

GLP-1 agonists act on intestinal receptors that are also involved in medication absorption. A study by Du et al. published in Cardiovascular Diabetology (2025) examined older adults on levothyroxine who initiated GLP-1 therapy and found that the interaction can alter thyroid hormone levels through changes in absorption and gastric emptying. GLP-1 slows gastric emptying, meaning the stomach takes longer to release the thyroid medication into the intestine where it would be absorbed.

In practice, this delay means that the levothyroxine you take on an empty stomach may not be fully absorbed if the GLP-1 was injected in the preceding or nearby hours. The result shows up in your test: TSH rises even though your thyroid medication dose has not changed.

With PeptPro, you log every GLP-1 injection with time and dose, and also record when you take your thyroid medication. When TSH changes from one quarter to the next, you have concrete data to show your endocrinologist.