Levothyroxine should not be taken with which substances?

• A. Caffeine
• B. Antacid, iron, or calcium
• C. Vitamin D
• D. Vitamin C

Answer: (B)

Levothyroxine absorption depends on reaching the small intestine in an unbound form, and certain minerals and antacids can prevent that. When taken with substances containing calcium or iron, or with calcium- or aluminum/magnesium-containing antacids, levothyroxine can bind to these minerals and form complexes that aren’t absorbed well. That lowers the drug’s effectiveness and can leave hypothyroid symptoms or require a higher dose.

That’s why the best choice is the option that includes an antacid, iron, or calcium, as these are known to interfere with absorption. Caffeine, vitamin D, and vitamin C don’t have the same clinically significant impact on levothyroxine absorption.

Practical takeaways: take levothyroxine on an empty stomach with water, about 30–60 minutes before breakfast, and separate it from calcium, iron, or antacids by about 4 hours. If calcium or iron supplements are needed, schedule them several hours apart from levothyroxine.