Several medicines can cause an underactive thyroid (hypothyroidism), including lithium, amiodarone, and some cancer treatments. These medicines affect thyroid function in different ways, and not everyone who takes them develops a problem. A healthcare professional may check thyroid blood tests before or during treatment. Do not stop or change a prescribed medicine on your own; discuss concerns with the clinician who manages it.
The thyroid gland produces hormones that help regulate energy use, body temperature, heart rate, and digestion. Some medicines interfere with hormone production or release, while others can cause inflammation that affects the gland. Recognizing these medication-related effects helps healthcare professionals monitor thyroid function and decide whether any changes in treatment are appropriate.
Which medications can cause hypothyroidism?
Several prescription medicines are associated with an underactive thyroid. The risk depends on the particular drug and a person's medical circumstances. The following groups are important to recognize because they may require thyroid function monitoring.
- Lithium: This medicine is used to treat certain mood disorders, including bipolar disorder. It can interfere with normal thyroid function and is a recognized cause of medication-related hypothyroidism. Thyroid changes may develop while someone is taking it.
- Amiodarone: This medicine treats certain abnormal heart rhythms. Its high iodine content can affect the thyroid, potentially causing either an underactive or an overactive gland.
- Interferons and certain cytokine treatments: These medicines affect immune activity and have been associated with thyroid inflammation and changes in thyroid hormone production. Their effects depend on the treatment and the individual.
- Immune checkpoint inhibitors: These cancer immunotherapy medicines help the immune system attack cancer cells. They can also lead to inflammation affecting hormone-producing glands, including the thyroid, and may result in hypothyroidism.
- Tyrosine kinase inhibitors: This group of targeted cancer medicines includes drugs such as sunitinib and sorafenib. These treatments can affect thyroid function, and some people develop an underactive thyroid during treatment.
- Bexarotene: This medicine is used to treat certain types of cutaneous T-cell lymphoma, a cancer involving the skin and immune cells. It can interfere with normal thyroid activity, so thyroid monitoring may be needed during treatment.
How do lithium and other mood-related medicines affect the thyroid?
Lithium is one of the established causes of drug-related thyroid dysfunction. It can affect the thyroid's ability to maintain normal hormone levels, and some people develop hypothyroidism during treatment. This is an important consideration when lithium is prescribed for a long-term mental health condition.
The American Thyroid Association identifies lithium, interferons, and certain immune-related medicines among the drugs associated with thyroiditis or other forms of thyroid dysfunction. Thyroiditis means inflammation of the thyroid gland. Depending on how the gland is affected, inflammation can lead to changes in hormone levels, including a period of hypothyroidism.
Developing a thyroid problem does not automatically mean lithium or another essential medicine must be discontinued. The clinician treating the underlying condition can assess thyroid results and symptoms, consider the benefits and risks of continuing treatment, and determine whether additional care is appropriate.
Why can amiodarone cause an underactive thyroid?
Amiodarone contains a large amount of iodine, which the thyroid uses to make hormones. Exposure to the iodine in this medicine can interfere with normal thyroid regulation. Some people develop hypothyroidism, while others experience an overactive thyroid. The effect cannot be predicted from symptoms alone.
Cambridge University Hospitals describes thyroid function testing before amiodarone treatment and regular testing during continued use. Its patient guidance describes checks every six months while taking the medicine. The monitoring schedule for an individual should follow the treating team's recommendations and take account of their clinical circumstances.
Which cancer treatments can affect thyroid hormone levels?
Immune checkpoint inhibitors
Immune checkpoint inhibitors are used to help the immune system recognize and attack cancer cells. In some people, these medicines cause immune-related inflammation that affects the thyroid or other hormone-producing glands. Thyroid function may change during treatment, so new symptoms or changes in blood test results should be discussed with the oncology team.
The National Cancer Institute identifies both overactive and underactive thyroid function among the possible effects of immune checkpoint inhibitors. People receiving these medicines should follow the monitoring plan provided by their cancer care team, even when they feel well.
Tyrosine kinase inhibitors
Tyrosine kinase inhibitors are targeted medicines used to treat several cancers, including some kidney and gastrointestinal cancers. Certain drugs in this group are associated with hypothyroidism, which may arise after treatment begins. The precise risk and timing differ among medicines, making ongoing assessment important.
The American Thyroid Association's review of tyrosine kinase inhibitor treatment discusses the development of hypothyroidism during cancer therapy and the role of thyroid testing before and during treatment. The oncology team can explain which checks are appropriate for a particular medicine.
Bexarotene
Bexarotene can reduce normal thyroid activity in people receiving it for certain skin-related cancers. The MedlinePlus drug information page for bexarotene notes that the medicine may stop the thyroid gland from working normally and describes monitoring for this effect. If thyroid changes occur, the treating clinician can determine whether additional treatment is needed while the cancer medicine is continued.
What symptoms may occur when a medicine affects the thyroid?
When thyroid hormone levels become lower than the body needs, everyday functions may slow down. Possible symptoms include:
- Ongoing tiredness or reduced energy
- Feeling unusually sensitive to cold
- Constipation
- Dry skin or thinning hair
- Unexpected weight gain
- Low mood or difficulty concentrating
- Muscle aches or joint discomfort
- Heavier or irregular menstrual periods
These experiences can have many explanations, including conditions unrelated to the thyroid or the medicine being taken. The MedlinePlus overview of hypothyroidism describes these symptoms and explains that thyroid function is evaluated using medical history and appropriate tests. Persistent or worsening symptoms are a reason to speak with a healthcare professional, particularly when they develop during treatment with a medicine known to affect the thyroid.
How do healthcare professionals check for medication-related hypothyroidism?
Assessment usually includes reviewing the medication list, the timing of treatment, symptoms, previous thyroid problems, and any earlier thyroid test results. A thyroid-stimulating hormone (TSH) blood test is commonly used to assess thyroid function. A free thyroxine (free T4) test may also be requested to help determine whether thyroid hormone levels are lower than expected.
Depending on the medicine, testing may be recommended before treatment begins and repeated during treatment. Monitoring is particularly relevant for medicines with established thyroid effects, such as amiodarone and certain cancer therapies. Comparing results over time helps clinicians identify changes and interpret them in relation to medication exposure and a person's overall health.
It is important to tell the healthcare professional about all prescription medicines, over-the-counter products, vitamins, and supplements being used. This information can help distinguish a medication-related change from other possible causes of abnormal thyroid results.
Does medication-related hypothyroidism go away?
The outcome depends on the medicine, the nature of the thyroid change, and the person's underlying thyroid health. Some drug-related changes may improve after the responsible medicine is stopped or changed under medical supervision. Other cases may persist and require continued monitoring or thyroid hormone replacement.
When a medicine is essential for controlling a heart rhythm disorder, managing a mental health condition, or treating cancer, stopping it without professional guidance can create significant risks. The treating clinician can weigh the importance of the medicine against its possible thyroid effects and decide whether monitoring, an adjustment in treatment, or thyroid hormone replacement is appropriate.
What should you do if your medicine may be affecting your thyroid?
Contact the healthcare professional responsible for your treatment if you develop persistent symptoms or have questions about changes in thyroid blood tests. Explain when symptoms began, which medicines you take, and whether the dose or treatment plan has recently changed. Bring previous test results when available, as they can help the clinician assess changes over time.
Do not independently stop a prescribed medicine, change its dose, or begin thyroid supplements to correct a suspected problem. The appropriate next step depends on the medicine and the clinical findings. With suitable monitoring and treatment when needed, many medication-related thyroid problems can be managed while the underlying condition continues to receive appropriate care.