Levothyroxine is a medication that can help treat hypothyroidism. It's a man-made hormone that acts like the natural hormone called T4. It's used to balance the levels of hormones in your body when your thyroid gland isn't working as it should.

Disclaimer

The content on this page is not medical advice and should be used for informational purposes only. Always consult your health care provider or pharmacist to determine what medication and dosage are right for you.

Reviewed by

Andrew Le, MD

Last updatedMay 19, 2023

Levothyroxine

Levothyroxine is a medicine that can help treat different types of hypothyroidism, which is when your thyroid gland doesn't work properly. It's used as a replacement therapy when your body doesn't make enough thyroid hormones on its own. This can happen due to different reasons, like problems with the thyroid gland itself or issues with other glands in your body that control the thyroid.

In addition to that, levothyroxine can also be used alongside other treatments like surgery and radioiodine therapy for a specific type of thyroid cancer called thyrotropin-dependent well-differentiated thyroid cancer. It helps manage the cancer by supporting the balance of hormones in your body.

Generic name

Levothyroxine, Eltroxin, Ermeza, Euthyrox, Levo-T, Levothroid, Levoxyl, Np Thyroid, Synthroid, Thyquidity, Tirosint, Unithroid

Rx or OTC

Available by prescription only

Black box warning

Not for treatment of obesity or weight loss.

When & How

Taking Levothyroxine Orally:

Take the tablets with a full glass of water.
It's best to take them 30 minutes to 1 hour before breakfast on an empty stomach. This way, you can avoid any choking or gagging.
It's important to consider the timing of your meals when taking Levothyroxine. If you regularly take it within 1 hour of certain foods that can affect absorption, talk to your doctor about whether any dose adjustments are needed.
When you take Levothyroxine, avoid using soybean products or other foods that can decrease its absorption.
If you're taking other medications that can interfere with the absorption of Levothyroxine, make sure to take them at least 4 hours apart from your Levothyroxine dose.

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Quick changes in emotions (emotional lability)
Headache
Being sensitive to heat (heat intolerance)
Trouble sleeping (insomnia)
Weakness in muscles (myasthenia)
Increased pressure around the brain (pseudotumor cerebri)
Hair loss (alopecia)
Losing weight without trying (weight loss)
Difficulty getting pregnant (infertility)
Shortness of breath (dyspnea)
Fever
Fast heart rate (tachycardia)
Shaking or trembling (tremor)
Weakening of bones (decreased bone mineral density)
Weak muscles (muscle weakness)
Muscle spasms
Hair loss (rash)
Hip joint problem in children (slipped capital femoral epiphysis)

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What is Levothyroxine?

Levothyroxine is a medicine that helps treat a condition called hypothyroidism. It is made in a lab and is a man-made version of a hormone called thyroxine (or T4), which is normally produced by the thyroid gland in our bodies. You may also hear it called L-thyroxine or Synthroid.

Hypothyroidism happens when the thyroid gland doesn't make enough thyroxine (T4) and another hormone called triiodothyronine (or T3). When these hormones are low, it can cause problems in our bodies. Some common symptoms of hypothyroidism are feeling tired, having a fast heartbeat, feeling sad or depressed, having dry skin and hair, getting muscle cramps, having trouble going to the bathroom, gaining weight, having trouble with memory, and feeling cold all the time.

In a healthy body, the thyroid gland makes thyroxine when it receives a signal from another hormone called Thyroid Stimulating Hormone (TSH). The thyroxine (T4) is then changed into its active form, triiodothyronine (T3). T3 is the hormone that has the most important effects in our bodies. It affects almost every cell, especially the heart. That's why the thyroid gland and its hormones are connected to how well our heart works.

Levothyroxine dosages

Tablets

Different Forms and Strengths of Levothyroxine:

Tablets (Levo-T, Levoxyl, Unithroid, generic):
- Available in strengths of 25mcg, 50mcg, 75mcg, 88mcg, 100mcg, 112mcg, 125mcg, 137mcg, 150mcg, 175mcg, 200mcg, and 300mcg.
Capsules (Tirosint, generic):
- Available in strengths of 13mcg, 25mcg, 50mcg, 75mcg, 88mcg, 100mcg, 112mcg, 125mcg, 137mcg, and 150mcg.
Oral solution:
- Available as Tirosint-SOL and Thyquidity.
- Tirosint-SOL strengths: 13mcg/mL, 25mcg/mL, 37.5mcg/mL, 44mcg/mL, 50mcg/mL, 62.5mcg/mL, 75mcg/mL, 88mcg/mL, 100mcg/mL, 112mcg/mL, 125mcg/mL, 137mcg/mL, 150mcg/mL, 175mcg/mL, and 200mcg/mL.
- Thyquidity strength: 20mcg/mL.
Injection, lyophilized powder for reconstitution:
- Available in strengths of 100mcg/vial, 200mcg/vial, and 500mcg/vial.
Injection, solution:
- Available as an injection with a strength of 100mcg/mL.

Dosage recommendations for different conditions:

Mild Hypothyroidism:
- The typical dose is 1.7 mcg per kg of body weight or 100-125 mcg per day, not exceeding 300 mcg per day.
- For individuals over 50 years old or under 50 years old with cardiovascular disease, the usual starting dose is 25-50 mcg per day, which may be adjusted by 12.5-25 mcg every 6-8 weeks.
- For individuals over 50 years old with cardiovascular disease, the usual starting dose is 12.5-25 mcg per day, which may be adjusted by 12.5-25 mcg every 4-6 weeks until the desired thyroid hormone level is reached.
Severe Hypothyroidism:
- The initial dose is 12.5-25 mcg per day, which may be adjusted by 25 mcg every 2-4 weeks as needed.
Subclinical Hypothyroidism:
- The initial dose is 1 mcg per kg of body weight per day, which may be sufficient. Alternatively, if replacement therapy is not initiated, the patient should be monitored annually for their clinical status.
Myxedema Coma:
- The initial intravenous (IV) dose is 300-500 mcg, followed by a daily dose of 50-100 mcg until the patient can switch to oral administration. In patients with cardiovascular disease, smaller doses may be considered.

Remember to always follow the dosage instructions provided by your doctor.

Side effects

Disclaimer

Medications may affect individuals differently. Usage of any medication may include side effects and other interactions. Here is a list of known common side effects and interactions. This list is not exhaustive -- there may be other side effects or interactions for this medication that are not listed here. In some cases, the likelihood of side effects or interactions may increase depending on dosage. It’s important to keep in mind that in extreme cases, other serious side effects, even death, may occur. Always consult your health care provider or pharmacist to determine what medication and dosage is right for you.

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Safety notes

Levothyroxine overdose is very rare, but it can happen accidentally, especially in children or older adults when taking the medication accidentally.

When this occurs, the levels of thyroxine (T4) and triiodothyronine (T3) in the body increase within 1 to 2 hours. In the initial stage of an overdose (around 6 to 12 hours after ingestion), common signs of toxicity may include trembling, fast heartbeat, high blood pressure, anxiety, and diarrhea. In rare cases, seizures, severe thyroid problems, sudden confusion, irregular heartbeats, and heart attacks can happen. Medical tests usually show higher levels of total T4 and T3 in the blood, lower levels of TSH, and increased levels of Free T4 and Free T3.

Any overdose of Levothyroxine needs immediate medical attention.

Once in professional care, here's what can be done to treat an acute overdose of levothyroxine:

Activated charcoal may be given to prevent the absorption of levothyroxine.
Cholestyramine can bind to thyroxine and help remove it from the body. It is usually taken as 4 grams orally every 8 hours.
Beta-blockers can be helpful in reducing the effects of thyroid hormone, especially on the heart. They can control fast heart rate and prevent irregular heart rhythms. Propranolol can also stop the conversion of T4 to T3 in the body.
Glucocorticoids like Dexamethasone (taken as a 4 mg oral dose) can decrease the conversion of LT4 to T3, which is the active hormone.
Propylthiouracil can be used to prevent the conversion of T4 to T3.
In severe cases, hemodialysis may be used, but its effectiveness is limited because T3 and T4 are mostly bound to proteins.
Hemoperfusion using activated charcoal is a more complex but effective procedure for reducing thyroxine levels. It is mainly reserved for adult patients with severe intoxication.
Cases of severe intoxication leading to a thyroid storm require treatment in a specialized intensive care unit.

It's important to remember that there is no specific antidote for levothyroxine overdose. Treatment options include gastric lavage (cleaning out the stomach), activated charcoal, cholestyramine, glucocorticoids, beta-blockers, propylthiouracil, and supportive measures. Again, if you suspect an overdose, seek immediate medical attention.

Levothyroxine interactions

Disclaimer

Tell your doctor if you are taking any of these medications or supplements

Some medications can interfere with the absorption of levothyroxine, making it less effective. To make sure you get the most out of your levothyroxine, follow these guidelines:

Take levothyroxine at least 4 hours apart from calcium carbonate, ferrous fumarate, and sevelamer. These medications can bind to levothyroxine and reduce its absorption.

Bile acid sequestrants like colesevelam, cholestyramine, and colestipol, as well as ion exchange resins like polystyrene sulfonate, can also reduce levothyroxine absorption. Take levothyroxine at least 4 hours before these drugs and keep an eye on your TSH levels.

If you're taking levothyroxine and orlistat together, your thyroid function may be affected due to reduced absorption. It's important to monitor your thyroid function regularly.

The absorption of levothyroxine relies on having enough stomach acid. Medications like sucralfate, antacids, and proton pump inhibitors can decrease stomach acid levels (hypochlorhydria) and affect levothyroxine absorption.

Furosemide can interfere with the protein binding of levothyroxine, leading to increased levels of free T4. Phenytoin and carbamazepine can decrease the protein binding of levothyroxine, potentially reducing total and free T4 levels. Although most patients remain clinically normal, it's important to monitor thyroid hormone levels closely.

Certain drugs can affect the levels of thyroid binding globulin (TBG) in your body. Estrogen, tamoxifen, clofibrate, opioids, mitotane, fluorouracil, and capecitabine can increase serum TBG levels. On the other hand, androgens can reduce TBG levels. These changes may require adjustments to your levothyroxine dose.

Propranolol and amiodarone can affect the conversion of T4 to T3 in your body. Carbamazepine, phenobarbital, and rifampin can increase levothyroxine metabolism, leading to lower serum concentrations. It's important to be aware of these interactions and consult your doctor for proper monitoring.

Certain medications like dopamine, glucocorticoids, and octreotide can decrease TSH concentration, while others like lithium, iodine, and sulphonamides can interfere with thyroid hormone synthesis. Make sure your healthcare provider is aware of all the medications you're taking.

If you're undergoing immune checkpoint inhibitor therapy (ICPi) such as pembrolizumab, nivolumab, or ipilimumab, you may develop hypothyroidism as a side effect. Your doctor may need to adjust your treatment.

Tell your doctor if you have any of these pre-existing conditions

Be cautious with soybean, papaya, and grapefruit, as they can impair the absorption of levothyroxine. It's best to avoid consuming these foods close to taking your levothyroxine.

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References

Eghtedari B, Correa R. Levothyroxine. [Updated 2022 Sep 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK539808/