Levothyroxine and Calcium Carbonate: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Calcium Carbonate
Levothyroxine
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Reduced levothyroxine absorption, reduced efficacy of levothyroxine and an increased risk of hypothyroidism
Interaction Deep Dive
The absorption and efficacy of levothyroxine was reduced when coadministered with calcium carbonate54 due to possible formation of an insoluble chelate 3, potentially resulting in hypothyroidism 2. Therefore, administration of levothyroxine and calcium should be separated by at least 4 hours 231.
Why it happens (mechanism)
Formation of an insoluble chelate
Literature reports
2 reports — tap to read
a) Three women with thyroid cancer who were receiving levothyroxine to suppress serum thyroid stimulating hormone (TSH) levels experienced reduced levothyroxine efficacy when calcium carbonate was ingested simultaneously. Patient 1 was receiving levothyroxine 125 mcg daily and had a baseline TSH level of 0.08 mU/L. When she started taking calcium carbonate for osteoporosis prevention, she experienced fatigue and weight gain. Five months later, her TSH level had increased to 13.3 mU/L. Three weeks after the calcium carbonate was discontinued, her TSH level had dropped to 0.68 mU/L. Patient 2 was stabilized on levothyroxine 325 mcg daily with a TSH level of 0.03 mU/L. When calcium carbonate 1000 mg daily was added to therapy, her TSH level increased to 7.31 mU/L. Separating the administration of levothyroxine and calcium carbonate caused her TSH level to decrease down to 0.13 mU/L. Patient 3 had a TSH level of 0.04 mU/L while on levothyroxine 175 mcg daily. Calcium carbonate 1000 mg ingested simultaneously as the levothyroxine caused her TSH level to rise to 8.4 mU/L. Separating the administration of the two drugs resulted in the TSH level decreasing to 0.02 mU/L 4.
b) A study involving twenty patients evaluated the effect of calcium carbonate on the absorption of levothyroxine. All patients that were included in this prospective cohort study were hypothyroid and were stable on a long-term regimen of levothyroxine. Subjects were to take 1200 mg/d of elemental calcium as calcium carbonate and levothyroxine concomitantly for 3 months. Mean free T4 level was significantly reduced from 17 pmol/L at baseline to 15 pmol/L during administration of calcium carbonate. After calcium discontinuation free T4 increased to 18 pmol/L. The mean total T4 level was significantly reduced during calcium treatment from 118 nmol/L at baseline to 111 nmol/L during administration of calcium, and increased to 120 nmol/L after calcium was discontinued. The mean thyrotropin level was significantly increased from 1.6 mIU/L to 2.7 mIU/L with calcium treatment, and then decreased to 1.4 mIU/L after calcium was discontinued. Thirteen of the twenty patients had a reduction in free T4 during calcium administration, and a change in free T4 did not occur in seven patients. Thyrotropin level was increased during calcium administration in thirteen of twenty patients, and seven patients had no change. Levothyroxine adsorbs to calcium carbonate in an acidic environment and consequently reduces the bioavailability of levothyroxine 5.
Common questions
Can I take Levothyroxine and Calcium Carbonate together?
Reduced levothyroxine absorption, reduced efficacy of levothyroxine and an increased risk of hypothyroidism Always confirm with your pharmacist or prescriber before making any change.
How serious is the Levothyroxine and Calcium Carbonate interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "rapid". Effects can appear quickly, often within about 24 hours of combining the drugs.
How strong is the evidence for this interaction?
The evidence is graded "established". Well documented — supported by controlled studies or strong clinical data.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Levothyroxine or Calcium Carbonate need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (5)
- Product Information: LEVO-T® oral tablets, levothyroxine sodium oral tablets. Neolpharma Inc, Caguas, PR, 2026. DailyMed
- Product Information: TIROSINT(R)-SOL oral solution, levothyroxine sodium oral solution. IBSA Pharma Inc. (per FDA), Parsippany, NJ, 2023. DailyMed
- Product Information: NOVOTHYROX(R) oral tablets, levothyroxine sodium oral tablets. Merck KGaA, Darmstadt, Germany, 2008. DailyMed
- Schneyer CR: Calcium carbonate and reduction of levothyroxine efficacy (letter). JAMA 1998; 279:750. PubMed
- Singh N, Singh PN, & Hershman JM: Effect of calcium carbonate on the absorption of levothyroxine. JAMA 2000; 283(21):2822-5. PubMed
Keep reading about Calcium Carbonate
Keep reading about Levothyroxine
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