Side effects of taking Synthroid, short and long-term

Synthroid (levothyroxine sodium) is a widely used prescription medicine for hypothyroidism. Knowing the side effects of Synthroid is important because hypothyroidism is a lifelong condition that will potentially need ongoing treatment. Synthroid is a manufactured, synthetic medicine that replaces your thyroid hormone, also called thyroxine or T4.

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Synthroid (or generic levothyroxine) is a thyroid hormone replacement therapy. It's used in people with thyroid problems, like Hashimoto's thyroiditis (which leads to an underactive thyroid gland) or hypothyroidism (low thyroid hormone levels). 

The active ingredient, levothyroxine, is also marketed under other brand names, including Tirosint, Levoxyl, Unithroid, and Levo-T. Synthroid can be used to treat some forms of thyroid cancer when combined with other therapies. However, it is not meant to treat thyroid diseases like goiters or benign thyroid nodules (DailyMed, 2019).

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

The goal of Synthroid is to restore your T4 (thyroxine) and TSH (thyroid-stimulating hormone) to normal levels by supplying your body with a synthetic version of the same hormone. Ideally, the correct dose of Synthroid mimics normal thyroid function.

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

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.

As you and your healthcare provider look for the correct dose of Synthroid or levothyroxine, you may experience side effects. These usually stem from the dosage not being quite right. If you're on a dose that's too low, you may experience the same symptoms you were feeling with your hypothyroidism. Inform your healthcare provider if you have any other medical conditions or take other medications to avoid potential drug interactions.

You can also experience some side effects if you're getting too much thyroid hormone. If your Synthroid dose is too high, it can lead 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 

This list does not contain all possible side effects. Consult with your healthcare provider if you have any questions about Synthroid side effects or would like to obtain more drug information.

Synthroid is considered a safe and effective therapy. It can take some trial and error to find the correct dose. Eventually, you and your healthcare provider will settle on an adequate amount 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 should be on the lowest possible dose to control your thyroid deficiency. That's why healthcare providers often start low and then slowly increase your dose, 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 (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 throughout 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 (meaning that you have an overactive thyroid gland). As you age, your thyroid requirements change, so a dose appropriate for you in your 40s may be too high when you reach your 60s. You will need lifelong monitoring to keep your thyroid hormone levels balanced. 

Researchers are looking into whether people who are getting levothyroxine overtreatment lose bone mass more quickly, increasing the risk of osteoporosis. A study of Korean women over 65 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 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 thyroid hormone blood tests. Monitoring your levels 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.

It's particularly important for women over age 50 to monitor their bone density since menopause also leads to a loss of estrogen, a critical bone protector. Your healthcare provider may also 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, you could end up with too much thyroid hormone. Hyperthyroidism can lead to heart problems, even heart failure, 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 appropriate treatment (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 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.

Anytime you think you may be experiencing side effects from your Synthroid, you should contact your healthcare provider. However, you can have serious side effects that require immediate medical attention, especially if you notice any of the following (DailyMed, 2019):

• Irregular heartbeat (arrhythmia)

• High blood pressure

• Heart failure

• Chest pain (angina)

• Heart attack (myocardial infarction)

• Cardiac arrest (heart stops working)

• Allergic reactions, including skin rash, hives, difficulty breathing, etc. 

The best way to avoid long-term side effects from taking Synthroid is to team up closely with your healthcare provider. They'll ensure you're on the optimal dose of Synthroid, especially as your requirements change over time. Get your thyroid levels checked regularly to avoid thyroid hormone levels that are too high or too low.

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Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

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Current version

April 29, 2021

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.