Levothyroxine with Calcium Carbonate Interaction Details

Brand Names Associated with Levothyroxine

Brand Names Associated with Calcium Carbonate

Medical Content Editor Dr. Brian Staiger, PharmD
Last updated Nov 05, 2023

Interaction Effect

Decreased levothyroxine absorption

Interaction Summary

The absorption and efficacy of levothyroxine was reduced when coadministered with calcium carbonate due to possible formation of an insoluble chelate. Therefore, administration of levothyroxine and calcium should be separated by at least 4 hours .

Severity

Moderate

Onset

Rapid

Evidence

Established

How To Manage Interaction

Calcium may form an insoluble chelate with levothyroxine; therefore, the administration of calcium carbonate and levothyroxine should be separated by at least four hours.

Mechanism Of Interaction

Formation of an insoluble chelate

Literature Reports

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 .

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 .

Levothyroxine Overview

• Levothyroxine is used to treat hypothyroidism (condition where the thyroid gland does not produce enough thyroid hormone). It is also used with surgery and radioactive iodine therapy to treat thyroid cancer. Levothyroxine is in a class of medications called hormones. It works by replacing thyroid hormone that is normally produced by the body.

• Without thyroid hormone, your body cannot function properly, which may result in poor growth, slow speech, lack of energy, excessive tiredness, constipation, weight gain, hair loss, dry, thick skin, increased sensitivity to cold, joint and muscle pain, heavy or irregular menstrual periods, and depression. When taken correctly, levothyroxine reverses these symptoms.

Calcium Carbonate Overview

• Calcium carbonate is a dietary supplement used when the amount of calcium taken in the diet is not enough. Calcium is needed by the body for healthy bones, muscles, nervous system, and heart. Calcium carbonate also is used as an antacid to relieve heartburn, acid indigestion, and upset stomach. It is available with or without a prescription.

• This medication is sometimes prescribed for other uses; ask your doctor or pharmacist for more information.

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Definitions

Severity Categories

Contraindicated

These drugs, generally, should not be used together simultaneously due to the high risk of severe adverse effects. Combining these medications can lead to dangerous health outcomes and should be strictly avoided unless otherwise instructed by your provider.

Major

This interaction could result in very serious and potentially life-threatening consequences. If you are taking this drug combination, it is very important to be under close medical supervision to minimize severe side effects and ensure your safety. It may be necessary to change a medication or dosage to prevent harm.

Moderate

This interaction has the potential to worsen your medical condition or alter the effectiveness of your treatment. It's important that you are monitored closely and you potentially may need to make adjustments in your treatment plan or drug dosage to maintain optimal health.

Minor

While this interaction is unlikely to cause significant problems, it could intensify side effects or reduce the effectiveness of one or both medications. Monitoring for changes in symptoms and your condition is recommended, and adjustments may be made if needed to manage any increased or more pronounced side effects.

Onset

Rapid: Onset of drug interaction typically occurs within 24 hours of co-administration.

Delayed: Onset of drug interaction typically occurs more than 24 hours after co-administration.

Evidence

Level of documentation of the interaction.

Established: The interaction is documented and substantiated in peer-reviewed medical literature.

Theoretical: This interaction is not fully supported by current medical evidence or well-documented sources, but it is based on known drug mechanisms, drug effects, and other relevant information.

How To Manage The Interaction

Provides a detailed discussion on how patients and clinicians can approach the identified drug interaction as well as offers guidance on what to expect and strategies to potentially mitigate the effects of the interaction. This may include recommendations on adjusting medication dosages, altering the timing of drug administration, or closely monitoring for specific symptoms.

It's important to note that all medical situations are unique, and management approaches should be tailored to individual circumstances. Patients should always consult their healthcare provider for personalized advice and guidance on managing drug interactions effectively.

Mechanism Of Interaction

The theorized or clinically determined reason (i.e., mechanism) why the drug-drug interaction occurs.

Disclaimer: The information provided on this page is for informational purposes only and should not be considered medical advice. Always consult with a qualified healthcare professional regarding your specific circumstances and medical conditions.

Where Does Our Information Come From?

Information for our drug interactions is compiled from several drug compendia, including:

• Gold Standard Drug Database
• Micromedex
• Lexicomp
• DrugBank

The prescribing information for each drug, as published on DailyMED, is also used. 

Individual drug-drug interaction detail pages contain references specific to that interaction. You can click on the reference number within brackets '[]' to see what reference was utilized.

The information posted is fact-checked by HelloPharmacist clinicians and reviewed quarterly.