Imatinib with Levothyroxine Interaction Details

Brand Names Associated with Imatinib

Brand Names Associated with Levothyroxine

Medical Content Editor Dr. Brian Staiger, PharmD
Last updated Nov 07, 2023

Interaction Effect

Decreased levothyroxine effectiveness and worsening of hypothyroidism

Interaction Summary

Initiation of imatinib therapy in post-thyroidectomy patients receiving levothyroxine has resulted in elevated levels of thyrotropin (TSH) and symptoms of hypothyroidism. While the exact mechanism for this interaction is unknown, it has been postulated that imatinib, similar to rifampin, induces the metabolism of levothyroxine which results in an increase in serum TSH levels. Therefore, when initiating imatinib treatment in hypothyroid, levothyroxine-substituted patients, particular post-thyroidectomy patients, an increase in their levothyroxine dose is advisable. Such patients should be monitored for signs and symptoms of hypothyroidism and the serum thyroxine and TSH levels should be routinely evaluated. A 2-fold increase in the levothyroxine dose before initiating imatinib treatment may be considered.

Severity

Moderate

Onset

Delayed

Evidence

Established

How To Manage Interaction

Hypothyroid patients, particularly post-thyroidectomy patients, receiving imatinib therapy may need an increase in their levothyroxine dose. Monitor the patient for signs and symptoms of hypothyroidism and monitor the serum thyroxine and TSH levels. Consider a 2-fold increase in levothyroxine dose before initiation of imatinib treatment.

Mechanism Of Interaction

Unknown

Literature Reports

A) Symptoms of hypothyroidism developed in post-thyroidectomy patients, on levothyroxine therapy, upon initiation of imatinib therapy. Patients (n=11) with gastrointestinal stromal tumor and medullary thyroid carcinoma were treated with imatinib for 6 months on average. Eight of the patients who underwent total thyroidectomy were also receiving levothyroxine and 3 patients had the thyroid in situ; all patients were euthyroid before treatment with imatinib. Within an average of 2 weeks of commencing imatinib therapy, the thyrotropin levels (TSH) increased to 384%+/-228% of the upper limit. The mean values of free thyroxine (fT4) and free tri-iodothyronine (fT3) decreased to 59% +/- 17% and 63% +/- 4% of the upper limits for fT4 and fT3, respectively, but remained within the reference range. These effects were observed only in post-thyroidectomy patients while patients with a normal functioning thyroid remained euthyroid after commencing imatinib therapy. A stepwise rise in levothyroxine dosage to an average of 206% (range, 100% to 350%) of the dose before imatinib treatment in all post-thyroidectomy patients reversed hypothyroidism only in 3 patients. In the remaining post-thyroidectomy patients, symptoms of hypothyroidism disappeared upon discontinuation of imatinib. Although the exact mechanism of this interaction between imatinib and levothyroxine was not determined, the investigators ruled out other probable causes of hypothyroidism and postulated that induction of levothyroxine metabolism by imatinib may be responsible for the observed hypothyroidism .

Imatinib Overview

• Imatinib is used to treat certain types of leukemia (cancer that begins in the white blood cells) and other cancers and disorders of the blood cells. Imatinib is also used to treat certain types of gastrointestinal stromal tumors (GIST; a type of tumor that grows in the walls of the digestive passages and may spread to other parts of the body). Imatinib is also used to treat dermatofibrosarcoma protuberans (a tumor that forms under the top layer of skin) when the tumor cannot be removed surgically, has spread to other parts of the body, or has come back after surgery. Imatinib is in a class of medications called kinase inhibitors. It works by blocking the action of the abnormal protein that signals cancer cells to multiply. This helps stop the spread of cancer cells.

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.

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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.