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Understanding Levothyroxine Side Effects

Levothyroxine pill next to a thyroid.

If you have low thyroid hormone levels, your healthcare professional may prescribe a medication called levothyroxine (Synthroid) to replace thyroid hormones produced by the thyroid gland in your body. Please continue reading to learn how this medicine works, as well as common levothyroxine side effects.

What is the thyroid gland and thyroid hormone?

The thyroid gland is an important gland located in the front of the neck. It secretes two main thyroid hormones, thyroxine (T4) and triiodothyronine (T3), that play a critical role in metabolism in adults, and brain development in infants. Your body, or more specifically all the cells in the body, need energy to carry on bodily function; metabolism breaks down the food you eat and transforms it into energy. Therefore, too little or too much thyroid hormone can lead to many health problems. 

Hyperthyroidism is a health condition when your thyroid gland makes too much thyroid hormone. There are many treatment approaches for this condition, but medications are typically the first step to treating someone with hyperthyroidism. Anti-thyroid medications used for hyperthyroidism (overactive thyroid gland) usually consist of medications such as methimazole (Tapazole) and propylthiouracil that function to prevent the thyroid gland from making too many thyroid hormones. 

On the other hand, hypothyroidism is when the thyroid gland doesn’t produce enough thyroid hormone causing impaired metabolism and leading to weight gain and other health problems. Treatment for hypothyroidism (underactive thyroid gland)  typically consists of taking thyroid hormone replacement tablets called levothyroxine sodium (brand names Levoxyl, Levo-T, Synthroid, Unithroid, Tirosint).

How does levothyroxine affect thyroid hormones?

Levothyroxine is the synthetic T4. In other words, levothyroxine replaces thyroxine hormone (T4) in people whose thyroid gland is not making enough of this thyroid hormone. Once levothyroxine is absorbed into the bloodstream, it’s converted to T3, the more active form, through the process explained below. 

Your thyroid gland releases T3 and T4, which work simultaneously to regulate the body’s metabolism. T4 is the active form, while T3 remains inactive, which requires an extra step where some body organs transform it to the active form, T3. 

Is levothyroxine used to treat thyroid cancer?

Yes, levothyroxine may be prescribed to treat thyroid cancer, along with radioactive iodine therapy and surgery. It is also used to treat or prevent enlarged thyroid gland (goiter).

What is the most common adverse effect of levothyroxine?

Common side effects of levothyroxine are listed below. Talk to your doctor if these side effects are severe or do not go away

  • Changes in appetite
  • Weight loss
  • Mood changes
  • Sleep problems
  • Headache
  • Heat sensitivity
  • Hair loss
  • Irregular heartbeats 
  • Leg cramps
  • Changes in menstrual periods

Some of the adverse effects of levothyroxine are more serious. Call your doctor immediately or get emergency medical attention if you develop the following symptoms and serious side effects after taking levothyroxine tablets:

  • Skin rash, hives, itching (allergic reactions)
  • Shortness of breath
  • Wheezing
  • Trouble breathing or swallowing
  • Swelling of the hands, feet, lower legs, or ankles
  • Flushing
  • Stomach pain and nausea

If you take too much medicine, call your local poison control center, the national poison control center at 1-800-222-1222, or get emergency medical care. Levothyroxine overdose symptoms may include chest pain, irregular heartbeat or racing heartbeat, shortness of breath, anxiety, irritability, nervousness, excessive sweating, confusion, sleep difficulties, loss of consciousness, and seizures.

How do I feel after taking levothyroxine?

Levothyroxine replaces thyroxine hormone to treat hypothyroidism (underactive thyroid). However, thyroid hormone levels are very sensitive to changes. So, taking levothyroxine can make your thyroid hormone levels too high, causing symptoms such as irregular heartbeats, mood changes, and heat intolerance, which can make you feel different types of symptoms that you didn’t experience before. Tell your healthcare provider if you develop any unusual symptoms after taking levothyroxine. Your doctor may adjust the dosing or discuss alternative treatment with the brand name medications of levothyroxine such as Synthroid, Levoxyl, and Tirosint. 

How long does it take for your body to adjust to levothyroxine? 

It takes the body a few days to a couple of weeks to adjust to levothyroxine. Any mild side effects should go away after this time. If your side effects are severe or persistent, tell your doctor.

How do you control the side effects of levothyroxine?

As mentioned above, the side effects of levothyroxine should go away as your body adjusts to the medicine. Tell your doctor or pharmacist if you have severe or persistent side effects after starting levothyroxine. You may be taking a larger dose of levothyroxine than you need to treat hypothyroidism (underactive thyroid). Your doctor may do a blood test to check thyroid hormones and adjust your dose based on the results. Taking a lower dosage of levothyroxine may help to reduce certain side effects. 

How long does it take for thyroid levels to stabilize?

Levothyroxine works right away after your body absorbs the medicine. However, it can take several weeks for you to get the full benefits of this medicine. So, it may be 4 weeks or longer before your symptoms start to improve. 

How to take levothyroxine?

Here are some things to keep in mind when taking levothyroxine.

Allergic Reactions

Before starting levothyroxine, tell your doctor if you have ever had an allergic reaction to levothyroxine, thyroid hormone, any of the active or inactive ingredients in levothyroxine formulations, or any other medications.

Medical History

Give a complete health history to your healthcare provider. Levothyroxine may not be safe for people with certain medical conditions, such as uncontrolled adrenal gland disorder, pituitary gland problems, heart disease, diabetes, liver disease, kidney disease, osteoporosis, swallowing difficulties, bleeding problems, anemia, or porphyria (a condition of the skin or nervous system caused by the accumulation of chemicals related to red blood cell proteins). 

Levothyroxine should not be used to treat obesity alone or with other drugs prescribed for losing weight. Taking large doses of levothyroxine to reduce body weight can lead to an increased risk of life-threatening problems.

Other Drugs

Levothyroxine can interact with many medications, including blood thinners, steroids, cough and cold medicines, diabetes medications, hormone replacement therapy containing estrogen, antacids, proton pump inhibitors, antidepressants, and more. Give your doctor or pharmacist a complete list of your other medicines, including prescription drugs, over-the-counter medications, vitamin supplements, and herbal products. This can help avoid dangerous drug interactions and serious side effects. 

Timing of other medications

Certain medications, such as ferrous sulfate (iron supplement) and calcium carbonate (Tums), should be taken at least 4 hours away from levothyroxine because they can interfere with its absorption. Other medicines should be taken at least 4 hours after a dose of levothyroxine. Examples of these medicines include cholesterol-lowering drugs like colesevelam (Welchol), colestipol (Colestid), and cholestyramine (Prevalite), and drugs prescribed for high potassium such as sodium polystyrene sulfonate (Kayexalate) and sevelamer (Renvela, Renagel).


Levothyroxine comes as a capsule or tablet to take by mouth on an empty stomach 30-60 minutes before your first meal of the day. Read the prescription label carefully and take levothyroxine exactly as prescribed. Your doctor may adjust your dose based on the results of blood tests to measure thyroid hormone levels.

Do not chew or crush the capsules. Remove the capsules from the packaging only when you are ready to take them. Swallow the tablets with a full glass of water to prevent them from getting stuck in your throat. You can crush the tablet and mix it with 1-2 teaspoons of water if you are giving levothyroxine to a child who cannot swallow tablets. However, do not mix the medicine in food or infant formula containing soybean.

Missed dose

If you forget to take a dose of levothyroxine, take it as soon as you remember. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not take a double dose to make up for a missed dose.

Foods and Beverages

The following food can reduce the amount of levothyroxine being absorbed: walnuts, soybeans, cotton seed meal, grapefruit, grapefruit juice, and high-fiber foods can affect how levothyroxine works. Before starting levothyroxine, talk to your doctor about eating or drinking these foods and beverages.


  1. https://www.ncbi.nlm.nih.gov/books/NBK279388/
  2. https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/treatment/#
  3. https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/treatment/#
  4. https://www.nhs.uk/medicines/levothyroxine/#
  5. https://www.mayoclinic.org/drugs-supplements/levothyroxine-oral-route/side-effects/drg-20072133?p=1
  6. https://medlineplus.gov/druginfo/meds/a682461.html