10 Levothyroxine Side Effects and How to Manage Them

Levothyroxine (Synthroid, Levoxyl, Tirosint) is a common thyroid hormone replacement medication. It’s used to treat hypothyroidism — a condition where your body doesn’t make enough thyroid hormone. Your thyroid gland makes these hormones to help regulate many body functions, including body temperature, energy levels, and mood.

Thyroid hormone replacement medications supply your body with extra thyroid hormones to help keep your body on track. They’re helpful for keeping symptoms of hypothyroidism well-managed. Some hypothyroid medications are natural (made from animals), while many others are synthetic (lab-made).

As one of the most commonly prescribed medications in the U.S., levothyroxine is considered a first-choice option for treating hypothyroidism. But, as with any medication, this synthetic drug comes with some drawbacks. Let’s explore several common levothyroxine side effects and what to expect if you’re taking this prescription.

Levothyroxine side effects at a glance

Levothyroxine works well to raise your thyroid hormone levels, but you may experience some side effects while taking it. Most side effects are due to changes in thyroid hormone levels that happen when you first start taking this thyroid medication (or while changing doses).

Common levothyroxine side effects:

• Excessive sweating

• Trouble sleeping or insomnia

• Fast heart rate

• Headache

• Irritability

• Larger-than-normal appetite

• Weight loss

• Heat intolerance

• Hair changes

• Diarrhea

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Rare but serious levothyroxine side effects:

• Tremors

• Mood changes

• Difficulty breathing or shortness of breath

• Bone fractures

• Harder-to-manage diabetes

• Chest pain

• Seizures

Side effects like these are more likely to happen when your dose is too high. They may mimic symptoms of hyperthyroidism — the opposite issue of hypothyroidism. Your prescriber will check your thyroid hormone levels regularly as a result. 

Here, we’ll dive deeper into 10 notable levothyroxine side effects you should know about.

1. Heat intolerance

Thyroid hormones play a role in maintaining body temperature. When there are more thyroid hormones in your body — such as through levothyroxine use — your core body temperature can go up. This makes you more sensitive to higher temperatures. This is called heat intolerance, where you feel hot when others around you might not.

There are a few things you can do to help manage heat intolerance. If you’re able to, keep inside room temperatures at a level that’s comfortable for you, or keep a fan nearby. You can also limit the amount of time you’re outside when it’s hot or humid.

EXPERT PICKS: WHAT TO READ NEXT

• Is your treatment working? After you start taking thyroid hormone medication, it may take a few weeks for your levels to improve. In the meantime, watch for these signs that your thyroid medication is working.

• Getting the most from your meds: Knowing how and when to take your thyroid medication can help you get the greatest benefit. Read these pharmacists’ top tips for taking medications like levothyroxine. 

• What’s it like to take levothyroxine? Learn about three people’s experiences starting this medication after being diagnosed with hypothyroidism.

2. Sweating

Sweating more than usual, especially on your palms or feet, is a common side effect of levothyroxine. Elevated levels of thyroid hormones can turn on your body’s “fight-or-flight” response, which can lead to sweating. Sweating can also be a consequence of heat intolerance, since you cool down by sweating.

To help with this, make sure to drink plenty of fluids. You lose body water when you sweat, which can raise your risk of dehydration. Over-the-counter (OTC) antiperspirants can also help with sweating. 

3. Irregular heartbeats or palpitations

Irregular heartbeats, such as a fast heart rate or heart palpitations, are also possible with this thyroid medication. Sometimes, this can feel like a racing or pounding heart.

There are some general steps you can take to help slow down your heart rate. It’s helpful to limit the amount of caffeine and energy drinks you consume, stay hydrated, and avoid excessive alcohol or tobacco use.

While a racing or pounding heart can happen when you’re taking levothyroxine, there are certain situations that need immediate medical attention. Get emergency treatment right away if your heart symptoms feel severe or occur with dizziness, chest pain, or shortness of breath. In rare cases, heart attacks are possible. 

4. Diarrhea

Diarrhea is another side effect of levothyroxine that you may experience. Elevated thyroid hormone levels can cause your gut to make more digestive juices and speed up how fast things move through your small intestine, causing diarrhea.

Small and temporary dietary changes, such as avoiding fatty or fried foods, may be helpful in managing or lessening diarrhea. And like sweating, you can lose body water when you have persistent diarrhea. Make sure to drink fluids to avoid dehydration if you experience diarrhea. 

When needed, certain OTC medications can provide short-term relief. But be sure to ask your pharmacist about potential levothyroxine interactions before adding additional medications or supplements to your regimen.

5. Weight changes

Changes in thyroid hormone levels are linked to adjustments in how much energy your body uses. It may also affect body weight by impacting your metabolism. Having higher thyroid hormone levels could lead to unintended weight loss over time.

But, there isn’t enough strong evidence to make conclusions about the effect of levothyroxine on body weight. It’s been suggested that weight loss from levothyroxine may be small and mainly from loss of water weight — not fat.

And although levothyroxine might affect your weight over time, it’s important to note that the medication isn’t used for weight management. It’s also important that you continue taking levothyroxine. Symptoms of hypothyroidism, such as weight gain, can return if you stop taking your medication.

6. Hair changes

Levothyroxine can possibly lead to hair thinning or weaker hair. It might also contribute to the darkening of gray or white hair. If it happens, it’s usually temporary and happens within the first few months of starting levothyroxine.

These changes in your hair are caused by shifts in your body’s thyroid hormone levels. This is because thyroid hormones affect your hair follicles and hair growth.

If hair loss continues or worsens after a few months, talk with your prescriber about possible hair-loss treatments that can fight back.

7. Tremors

Tremors (shakiness) in the hands can happen while taking levothyroxine. Thyroid hormones activate the nervous system, which is believed to cause tremors. 

Let your prescriber know if this is happening to you. Based on your situation, they may be able to recommend specific tips for your symptoms. The best way to manage tremors varies by person.

8. Mood changes

Levothyroxine may cause mood changes. This is because thyroid hormones affect chemical messengers in areas of the brain that control emotion and mood.

If you notice you’re feeling more nervous, irritable, or anxious while taking levothyroxine, talk to your prescriber. They can recommend different ways to help promote feelings of calm, such as cognitive behavioral therapy (CBT) or stress-relief techniques. 

9. Trouble sleeping

Levothyroxine has been linked to sleep problems (insomnia). Your thyroid hormone levels can affect your quality of sleep, including how well you fall or stay asleep. 

Some tips to help you sleep better include getting regular exercise during the day and keeping your bedroom cool and dark at bedtime. Having a consistent sleep and wake time — and avoiding too much daytime napping or caffeine — can also help improve your sleep hygiene.

10. Bone loss and fractures

Levothyroxine can raise your risk of bone loss. Thyroid hormones affect how strong your bones are. It can also lower your bone mineral density (BMD), which measures the strength of your bones.

Both bone loss and low BMD can raise your risk of bone fractures. Your risk may be higher if your levothyroxine dose is too high, you’re a woman who has gone through menopause, or you’re at risk of osteoporosis.

Some ways to help support healthy bones include eating a calcium-rich diet and getting enough vitamin D. Also, make sure to get enough exercise, which can help boost your bone health.

If you experience bone pain or a potential broken bone while taking levothyroxine, let your prescriber know right away.

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Written by:

Taylor K. Clarke, PharmD

Taylor K. Clarke, PharmD, has 8 years of pharmacy experience working in community practice, hospital pharmacy, and ambulatory care pharmacy. Taylor's commitment to patient education motivated her to pursue health communications as a pharmacist.

Edited by:

Joshua Murdock, PharmD, BCBBS

Joshua Murdock, PharmD, BCBBS, is a licensed pharmacist in Arizona, Colorado, and Rhode Island. He has worked in the pharmacy industry for more than 10 years and currently serves as a pharmacy editor for GoodRx.

Reviewed by:

Austin Ulrich, PharmD, BCACP

Austin Ulrich, PharmD, BCACP, is a board-certified ambulatory care clinical pharmacist. In his early career, he provided direct patient care in a variety of settings, including hospital and community pharmacies, and in a primary care clinic as a clinical pharmacist.

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