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

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

Since most people with hypothyroidism have the condition for life with potentially life-long treatment, it makes sense to delve into the possible long-term side effects. 

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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 some forms of thyroid cancer when combined with other therapies (DailyMed, 2019).

There's no cure for hypothyroidism, a condition that affects approximately 5% of the population and impacts your entire body (Chiovato, 2019). That said, thyroid replacement medicines like levothyroxine can help you manage the condition and lessen signs and symptoms of hypothyroidism, such as tiredness and fatigue, sensitivity to cold, hair loss, and weight gain.

The goal of levothyroxine, and its brand name counterpart Synthroid, is to restore your T4 and TSH (thyroid-stimulating hormone) to normal levels by supplying your body with a synthetic version of the same hormone. Essentially, the right dose of Synthroid mimics normal thyroid function.

Most sources indicate it takes four to eight weeks to start feeling better on Synthroid (DailyMed, 2019). Your healthcare provider will check your TSH blood test approximately six weeks after starting the medicine to make sure your thyroid hormone levels have returned to normal—this means that you're taking the right amount of thyroid hormone. 

Synthroid is usually taken once a day, ideally on an empty stomach. To maximize effectiveness, avoid taking it with prescription or over-the-counter antacids. 

One thing to be aware of is that the U.S. Food and Drug Administration (FDA) has issued a black box warning for levothyroxine: You should not use levothyroxine for weight loss. Large doses of levothyroxine can lead to serious and potentially life-threatening adverse effects (DailyMed, 2019).

As you and your healthcare provider look for the right dose of Synthroid or levothyroxine, you may experience side effects—these usually stem from the dosage not being quite right. Symptoms of too low a dose of thyroxine are basically what you've been feeling with your hypothyroidism.

However, it's also possible to have certain adverse effects because you're getting too much thyroid hormone—your Synthroid dose is too high, leading to a condition known as hyperthyroidism.

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

• Fast or abnormal heart rate

• Headache

• Chest pain or shortness of breath

• Feeling extra irritable or nervous or shaky (tremors)

• Trouble sleeping

• Excessive sweating and unable to deal with feeling hot (heat intolerance)

• Changes in your appetite and possible diarrhea

• Vomiting or weight changes

• Leg cramping

• Muscle weakness

• Fever for no apparent reason

• Changes in your menstrual period. 

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

• Irregular heartbeat (arrhythmia)

• High blood pressure

• Heart failure

• Chest pain (angina)

• Heart attack (myocardial infarction)

• Cardiac arrest (heart stops working)

Get immediate medical attention if you notice any of these symptoms or you have side effects that worsen.

Synthroid (and levothyroxine) is considered a safe and effective therapy. Eventually, you and your healthcare provider will settle on an effective dose for the long haul. People take these medicines for decades—usually for a lifetime.

Because of this, it's a good idea to confirm with your healthcare provider that you're on the right amount of thyroid hormone. Ideally, you want to be on the lowest possible dose to control your thyroid deficiency, which is why healthcare providers often start low and then titrate up the amount depending on your response (Chiovato, 2019). 

There are two potential side effects from long-term use that you may want to discuss with your healthcare provider: bone-thinning (or osteoporosis) and heart-related issues. 

Osteoporosis

When bones thin out and lose mass, they become weaker and more likely to fracture or break. Over time, a condition called osteoporosis can develop. Osteoporosis isn't necessarily an inevitable part of aging: a healthy diet and weight-bearing exercises through life can help keep your skeletal bones healthy. 

High levels of levothyroxine can speed up bone loss, essentially replicating what would happen if you were hyperthyroid. As you age, your thyroid requirements change, so a dose right for you in your 40s may be too high when you reach your 60s. Lifelong monitoring is needed to keep your thyroid hormone levels balanced. Researchers are still looking into whether people who are getting levothyroxine overtreatment lose bone mass more quickly, increasing the risk of osteoporosis. 

A 2014 study of women over 65 in Korea looked at the relationship between a higher dose of levothyroxine and the risk of bone fractures. They found that women who were already at an increased risk for osteoporosis experienced more bone fractures if they took higher doses of levothyroxine (Ko, 2014). 

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

One way to decrease the likelihood of developing osteoporosis is to have regular TSH blood tests—this helps you keep your levothyroxine dose in line with your thyroid hormone requirement. Another option is to get regular bone mineral density tests to check for any changes to bone density over time. 

This is particularly important for women over age 50 since menopause also leads to a loss of (bone-protective) estrogen. Lastly, your healthcare provider may recommend calcium supplements or a regimen of weight-bearing exercises to improve your bone health.

Heart problems

It's well-established that abnormal levels of thyroid hormone can affect the heart. Researchers continue to study the long-term impact of Synthroid and other thyroid replacement medicines on the heart (Udovcic, 2017). 

If you don't carefully monitor your hormone levels and end up with too much thyroid hormone (hyperthyroidism), this can lead to heart problems over time. You may experience a racing heartbeat, chest pain, and tightness, or even a heart attack. Fast or irregular heartbeats (like atrial fibrillation), palpitations, and abnormal heart rhythms need to be treated appropriately. High levels of thyroid hormone can also lead to heart failure (Klein, 2007). 

On the other hand, if your levothyroxine dose is too low, you increase your risk of high cholesterol and heart disease, potentially leading to heart attacks and strokes (Klein, 2007). So you can see that optimal heart health depends on a balanced thyroid hormone system.

The best way to avoid long-term side effects from taking Synthroid is to team up closely with your healthcare provider to make sure you're on the optimal dose of Synthroid, especially as your requirements change over time. Not too low and not too high. Be consistent. And get your TSH tested regularly. 

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Current version

October 09, 2020

Written by

Andrea Pierce

Fact checked by

Chimene Richa, MD

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About the medical reviewer

Chimene Richa, MD

Dr. Richa is a board-certified Ophthalmologist and medical writer for Ro.