Calcium supplements and calcium-containing foods reduce the absorption of levothyroxine, potentially causing inadequate thyroid hormone levels and a return of hypothyroid symptoms. This interaction is common and often overlooked, leading to unnecessary dose increases.
How This Interaction Works (Mechanism)
Calcium ions form insoluble complexes with levothyroxine (T4) in the acidic environment of the stomach, reducing its absorption from the small intestine. Studies show that calcium carbonate can reduce levothyroxine absorption by 20 to 25 percent. This can raise TSH levels and produce hypothyroid symptoms despite taking an appropriate levothyroxine dose. The interaction is dose-dependent, with higher calcium doses causing greater absorption reduction.
Source: American Thyroid Association Clinical Practice Guidelines, 2025
Separate levothyroxine and calcium supplements by at least 4 hours. Take levothyroxine first thing in the morning on an empty stomach with water, and take calcium supplements at lunch or dinner. This spacing applies to all calcium-containing products, including antacids (Tums), calcium-fortified foods, and dairy products. Have your TSH checked 6 to 8 weeks after making any changes to your medication schedule.
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Sources and standards behind this page
What a drug interaction does, and how strongly it is supported, is documented in public labeling and federal safety references rather than in any one summary, and the sources below are the primary ones. They are linked so a reader can check a claim without taking this page's word for it.
- DailyMed carries the current FDA labeling for every marketed drug product, including the warnings and drug interaction sections.
- Drugs@FDA is the federal database of approved drug products and their approval documents.
- FDA: Drug interactions, what you should know explains how drug-drug, drug-food and drug-condition interactions are described on labels.
- FDA Drug Safety Communications publishes the agency's safety alerts and label changes as they are issued.
- FDA MedWatch is the program for reporting and reviewing adverse events, including suspected interactions.
- openFDA provides public access to FDA labeling and adverse event data.
- MedlinePlus drug information is the National Library of Medicine's consumer reference for what a medicine is and how it is taken.
- NIAAA: Harmful interactions documents which medicines interact with alcohol and how.
- NCCIH: Herb-drug interactions summarises the evidence on interactions between herbal products and medicines.
- National Library of Medicine maintains the drug and health databases the sources above draw on.
This page describes how this interaction works and where it is documented. It is not medical advice: a prescriber or pharmacist decides what applies to a particular patient and medication.
Frequently Asked Questions
Levothyroxine and Calcium have a moderate interaction that requires caution. While some patients may take both under medical supervision, the combination can cause adverse effects. Talk to your doctor before combining these medications, and do not adjust doses on your own.
Calcium supplements and calcium-containing foods reduce the absorption of levothyroxine, potentially causing inadequate thyroid hormone levels and a return of hypothyroid symptoms. This interaction is common and often overlooked, leading to unnecessary dose increases.
Yes. Anytime you are taking multiple medications, supplements, or substances, you should inform your doctor and pharmacist. They can evaluate your specific risk factors (age, kidney function, other medications, medical conditions) and determine whether the Levothyroxine and Calcium combination is safe for you, or whether adjustments are needed.
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