DrugPair interaction guide

Levothyroxine and Calcium

Levothyroxine (Synthroid) is a thyroid hormone replacement. Calcium supplements can bind to levothyroxine in the gut and prevent it from being absorbed properly.

Written by DrugPair Editorial Team. Updated 16 July 2026.

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Moderate interaction

Use with caution — ask your doctor or pharmacist.

What you need to know

Calcium-containing products can reduce levothyroxine absorption when taken too close together. Timing instructions can vary by formulation and product, so follow the prescription label and confirm the spacing for supplements, antacids, and calcium-fortified products with a pharmacist.

What to do

Take levothyroxine exactly as directed and use a consistent routine with plain water.

Follow the food-timing instructions on your label; ask a pharmacist if your formulation or routine differs.

NIH MedlinePlus advises separating calcium products from levothyroxine by at least 4 hours; confirm the timing for your exact products with a pharmacist.

When to call your doctor

Symptoms of underactive thyroid returning: fatigue, weight gain, feeling cold, constipation.

If your TSH level has changed without a dose adjustment.

Sources

Levothyroxine: Drug InformationNIH MedlinePlus. Accessed 2026-07-16.Drug Interactions: What You Should KnowU.S. Food and Drug Administration. Accessed 2026-07-16.

Source databases can disagree or omit an interaction. Report an error.

DrugPair provides educational safety information only and is not a substitute for professional medical advice. Always ask a doctor or pharmacist before changing medicines, supplements, food, drinks, or prescription timing.

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