What are the long-term side effects of taking Synthroid? | Ro (2023)


  1. What is Synthroid used for?
  2. What are the short term side effects of Synthroid?
  3. What are the long-term side effects of taking Synthroid?
  4. Application

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If you have any medical questions or concerns, talk to your doctor. Health Guide articles are backed by peer-reviewed research and information from medical associations and government agencies. However, they are not a substitute for professional medical advice, diagnosis or treatment.

Levothyroxine sodium, sold under brand names such as Synthroid, Unithroid, Levoxyl, and Tirosint, is a commonly used prescription medication for hypothyroidism. These man-made, synthetic drugs replace the hormone, thyroxine or T4, that your thyroid would normally produce.

Since most people with hypothyroidism are able to live with potentially ongoing treatment, it makes sense to investigate potential long-term side effects.

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What is Synthroid used for?

Synthroid (or generic levothyroxine) is used for thyroid hormone replacement therapy in people with low thyroid hormone levels (Hypothyroidism). It can also be used to treat certain forms of thyroid cancer in combination with other treatments (DailyMed, 2019).

There is no cure for hypothyroidism, a condition that affects about 5% of the population and affects the entire body (Chiovato, 2019). However, thyroid replacement drugs such as levothyroxine can help manage the condition and reduce the symptoms of hypothyroidism, such as fatigue and exhaustion, sensitivity to colds, hair loss, and weight gain.

The purpose of levothyroxine and its Synthroid counterpart is to bring T4 and TSH (thyrotropic hormone) back to normal levels by providing the body with a synthetic version of the same hormone. Basically, the correct dose of Synthroid mimics normal thyroid function.

Most sources say it takes four to eight weeks to feel better on Synthroid (DailyMed, 2019). Your doctor will check your TSH blood test about six weeks after starting treatment to make sure your thyroid hormone levels are back to normal. This means you are getting the right amount of thyroid hormone.

Synthroid is usually taken once a day, preferably on an empty stomach. To increase effectiveness, avoid using prescription or over-the-counter antacids.

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What are the short term side effects of Synthroid?

It should be noted that the U.S. Food and Drug Administration (FDA) has issued a black box warning for levothyroxine: Levothyroxine should not be taken for weight loss. High doses of levothyroxine can lead to serious and potentially life-threatening side effects (DailyMed, 2019).

As you and your doctor search for the right dose of Synthroid or levothyroxine, you may feel some discomfortside effects— are usually the result of an incorrect dose. The symptoms of low thyroxine are basically what you experienced with hypothyroidism.

However, it is also possible that some side effects may occur due to taking too much thyroid hormone - the dose of Synthroid is too high, leading to a condition called hyperthyroidism.

Be sure to seek medical advice if you experience any of the following side effects so that your dose can be adjusted (DailyMed, 2019).

  • Fast or abnormal heartbeat
  • Headache
  • Chest pain or difficulty breathing
  • Increased irritability or nervousness or tremors (shaking)
  • Problems with sleeping
  • Excessive sweating and inability to cope with the feeling of heat (heat intolerance)
  • Changes in appetite and possible diarrhea
  • Vomiting or weight changes
  • Leg cramps
  • Weak muscles
  • Fever for no apparent reason
  • Changes in your period.

If the dose of levothyroxine is too high, serious side effects can occur, mainly affecting the heart, including (DailyMed, 2019):

  • Irregular heart rate (arrhythmias)
  • High blood pressure
  • Cardiac arrest
  • chest pain (angina)
  • Heart attack (myocardial infarction)
  • Cardiac arrest (heart stops beating)

Get medical help right away if you notice any of these symptoms or experience any side effects that get worse.

Synthroid side effects, short and long term

Dry Last updated: April 29, 2021 Read for 6 minutes

What are the long-term side effects of taking Synthroid?

Synthroid (and levothyroxine) is considered a safe and effective therapy. Ultimately, you and your doctor will decide on the long-term effective dose. People have been taking these drugs for decades - usually for life.

That's why it's a good idea to talk to your doctor about whether you're taking the right amount of thyroid hormone. Ideally, you should take the lowest dose possible to get your thyroid deficiency under control. Therefore, doctors often start with a low dose and then adjust the dose based on response (Chiovato, 2019).

There are two possible side effects of long-term use that should be discussed with your doctor: bone thinning (or osteoporosis) and heart problems.


When bones become thinner and lose weight, they become weaker and more likely to fracture or fracture. Over time, a condition called osteoporosis can develop. Osteoporosis is not necessarily an inevitable part of aging: a healthy diet and lifelong weight-bearing exercise can help maintain healthy skeletal bones.

High levels of levothyroxine can accelerate bone loss, essentially reproducing what would happen if you had hyperthyroidism. As you age, your thyroid needs change, so the dose that works for you at 40 may be too high at 60. Lifelong monitoring is necessary to maintain balanced thyroid hormone levels. Researchers are still investigating whether people overtreated with levothyroxine lose bone mass faster, increasing their risk of osteoporosis.

A study conducted in Korea in 2014 over the age of 65 looked at the association between a higher dose of levothyroxine and the risk of bone fractures. They found that women who were already at increased risk of osteoporosis had more bone fractures when they took higher doses of levothyroxine.Namely, in 2014).

Another study found that if TSH levels were within the normal range, there was no increased risk of fractures (Thayakaran, 2019). So if you are taking the right dose of levothyroxine for your body, you are less likely to develop osteoporosis.

One way to reduce your risk of developing osteoporosis is to regularly test your TSH blood. This will help keep your levothyroxine dose in line with your thyroid hormone needs. Another option is to have your bone mineral density checked regularly to see how your bone density changes over time.

This is especially important for women over 50, as menopause also leads to a loss of estrogen (which protects bones). Finally, your doctor may recommend a calcium supplement or a weight-bearing exercise program to help improve bone health.

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heart problems

It is well known that abnormal levels of thyroid hormones can affect the heart. Researchers continue to study the long-term effects of Synthroid and other thyroid replacement drugs on the heart (Udovčić, 2017 (monographs).).

If you don't monitor your hormone levels carefully and get too much thyroid hormone (hyperthyroidism), it can lead to heart problems over time. You may experience a rapid heartbeat, chest pain and tightness, and even a heart attack. Fast or irregular heartbeat (egatrial fibrillation), palpitations and arrhythmias should be treated appropriately. High levels of thyroid hormones can also lead to heart failure (Klein, 2007. (encyclopedia entry).).

On the other hand, if your levothyroxine dose is too low, you increase the risk of intoxicationcholesterolIheart diseasewhich can lead to myocardial infarction and stroke (Klein, 2007). So you can see that optimal heart health depends on a balanced thyroid hormone system.


The best way to avoid the long-term side effects of taking Synthroid is to work closely with your doctor to ensure you are taking the optimal dose of Synthroid, especially as your needs change over time. Not too low and not too high. Be consistent. And check your TSH regularly.


  1. Chiovato, L., Magri, F., and Carlé, A. (2019). Hypothyroidism in context: where we've been and where we're going.Progress in therapy,36(S2), 47-58. doi: 10.1007/s12325-019-01080-8. retrieved fromhttps://pubmed.ncbi.nlm.nih.gov/31485975/
  2. DailyMed. (2019). Levothyroxine sodium tablets. Retrieved October 9, 2020https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=fce4372d-8bba-4995-b809-fb4e256ee798
  3. Jonklaas, J., Bianco, AC, Bauer, A. J., Burman, K. D., Cappola, A. R., Cell, F. S. et al. (2014). Hypothyroidism Treatment Guidelines: Developed by the American Thyroid Association Thyroid Hormone Replacement Task Force.Thyroid,24(12), 1670-1751. doi: 10.1089/thousand 2014.0028. retrieved fromhttps://pubmed.ncbi.nlm.nih.gov/25266247/
  4. Klein, I. and Danzi, S. (2007). Thyroid and heart diseases.Issue, 116(15), 1725-1735. doi: 10.1161/CIRCULATIONAHA.106.678326. retrieved fromhttps://pubmed.ncbi.nlm.nih.gov/17923583/
  5. Ko, YJ, Kim, JY, Lee, J, Song, HJ, Kim, JY, Choi, NK, et al. (2014). Levothyroxine dose and fracture risk according to osteoporosis status in older women.Journal of Preventive Medicine and Public Health,47(1), 36-46. doi: 10.3961/jpmf.2014.47.1.36/. retrieved fromhttps://pubmed.ncbi.nlm.nih.gov/24570805/
  6. Thayakaran, R., Adderley, N.J., Sainsbury, C., Torlinska, B., Boelaert, K., Šumilo, D. et al. (2019). Thyroid replacement therapy, thyroid stimulating hormone levels, and long-term health outcomes in patients with hypothyroidism: a longitudinal study.BMJ (Clinical Trials ed.), 366, l4892. doi: 10.1136/bmj.l4892. pobrane zhttps://pubmed.ncbi.nlm.nih.gov/31481394/
  7. Udovcic, M., Pena, R.H., Patham, B., Tabatabai, L., and Kanker, A. (2017). Hypothyroidism is rare.Metodist DeBakey Cardiovascular Journal,13(2), 55-59. doi: 10.14797/mdcj-13-2-55. retrieved fromhttps://pubmed.ncbi.nlm.nih.gov/28740582/

What are the long-term side effects of taking Synthroid? | Ro (6)

Chimene Richa, dr. med

Dr. Chimene Richa is a board certified ophthalmologist and senior medical author/reviewer at Ro.


What are the long-term effects of being on Synthroid? ›

Taking too much or too little SYNTHROID may lead to negative effects on growth and development, heart function, bone health, reproductive function, mental health, digestive function, and changes in blood sugar and cholesterol metabolism in adult or pediatric patients.

How long can you be on Synthroid? ›

Generally, thyroid replacement treatment is to be taken for life. SYNTHROID should not be used to treat noncancerous growths or enlargement of the thyroid in patients with normal iodine levels, or in cases of temporary hypothyroidism caused by inflammation of the thyroid gland (thyroiditis).

Can Synthroid damage your heart? ›

The use of Synthroid may cause heart problems, such as arrhythmia and heart palpitations. Arrhythmia is an irregular heart rate or rhythm.

What is the long-term damage from levothyroxine? ›

Yes, it's safe to take levothyroxine for a long time, even many years. However, high doses of levothyroxine over a long time can sometimes cause weakening of the bones (osteoporosis). This should not happen if you are on the right dose. It's important to have regular blood tests to make sure your dose is not too high.

Is there a downside to taking thyroid medication? ›

Common side effects of levothyroxine include heat intolerance, a fast heart rate, and diarrhea. More serious levothyroxine side effects are also possible. If you experience side effects like tremors or mood changes, talk to your healthcare provider as soon as you can.

Can Synthroid be harmful? ›

Taking too much or too little SYNTHROID may lead to negative effects on growth and development, heart function, bone health, reproductive function, mental health, digestive function, and changes in blood sugar and cholesterol metabolism in adult or pediatric patients.

Can you eventually stop taking Synthroid? ›

Thyroid medications like levothyroxine (Synthroid) are used to treat low thyroid levels (hypothyroidism). If you have borderline hypothyroidism, you may be able to stop your thyroid medication after some time. But those with more obvious hypothyroidism often need thyroid medications long term.

What vitamins should you avoid if you have hypothyroidism? ›

Avoid taking your thyroid hormone at the same time as:

Iron supplements or multivitamins containing iron. Calcium supplements. Antacids that contain aluminum, magnesium or calcium. Some ulcer medications, such as sucralfate (Carafate)

Should I take thyroid medication forever? ›

An underactive thyroid is a lifelong condition, so you'll usually need to take levothyroxine for the rest of your life. If you're prescribed levothyroxine because you have an underactive thyroid, you're entitled to a medical exemption certificate.

What is an alternative to Synthroid? ›

Armour Thyroid and levothyroxine (Synthroid, Levoxyl, Tirosint) are two thyroid replacement medications. Armour Thyroid is made from animal sources, whereas levothyroxine is synthetic (man-made). Levothyroxine is the first-choice treatment for hypothyroidism.

Is Synthroid cancerous? ›

They found that that women of all ages are up to 300% more likely to contract lung cancer if they also use Synthroid, but the research team stops short of stating that the drug causes lung cancer outright.

How do I get off Synthroid? ›

Or cut the original dose by 25 mcg every two months, stopping no later than six months [2]. In general, those who are on high doses of thyroid hormone or who have been taking their medication for a longer amount of time will need to wean off more slowly.

What are 6 adverse effects of levothyroxine? ›

Levothyroxine may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away:
  • weight gain or loss.
  • headache.
  • vomiting.
  • diarrhea.
  • changes in appetite.
  • fever.
  • changes in menstrual cycle.
  • sensitivity to heat.
Feb 15, 2019

What organs does levothyroxine affect? ›

Once in the bloodstream, thyroxine travels to the organs, like the liver and kidneys, where it is converted to its active form of triiodothyronine. Thyroxine plays a crucial role in heart and digestive function, metabolism, brain development, bone health, and muscle control.

What organ systems are affected by levothyroxine? ›

Organ Systems Involved

Thyroid hormone affects virtually every organ system in the body, including the heart, CNS, autonomic nervous system, bone, GI, and metabolism.

Can long-term use of Synthroid cause osteoporosis? ›

Too much thyroid medication or poor management of the medication can cause increased bone loss and risk for fracture. The development of osteoporosis can occur with using thyroid hormones for several years. This is a risk for most consumers who are put on Synthroid or other thyroid meds for life.

Can you ever stop taking Synthroid? ›

Thyroid medications like levothyroxine (Synthroid) are used to treat low thyroid levels (hypothyroidism). If you have borderline hypothyroidism, you may be able to stop your thyroid medication after some time. But those with more obvious hypothyroidism often need thyroid medications long term.

What are the symptoms of overmedication with Synthroid? ›

The signs and symptoms of thyroid overmedication include anxiety, diarrhea, depression, elevated heartbeat, elevated blood pressure, fatigue, irritability, difficulty concentrating, difficulty sleeping, being overheated, and unprompted or unintentional weight loss.

Can too much Synthroid cause liver damage? ›

When given in high doses, thyroid hormone preparations can cause mild serum enzyme elevations. In addition, standard doses of levothyroxine have been linked to rare instances of mild, immunoallergic liver injury.


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