Levothyroxine Side Effects

Medically reviewed by Drugs.com. Last updated on Mar 11, 2024.

Applies to levothyroxine: oral capsule liquid filled, oral solution, oral tablet.

Other dosage forms:

• injection powder for solution

Precautions

It is very important that your doctor check the progress of you or your child at regular visits. This will allow your doctor to see if the medicine is working properly and to decide if you should continue to take it. Blood tests will be needed to check for unwanted effects.

Levothyroxine should not be used for the treatment of obesity or for the purpose of losing weight. This medicine is not effective for weight reduction. If taken in large amounts, levothyroxine may cause serious unwanted effects.

Hypothyroidism can sometimes cause infertility in men and women. Levothyroxine should not be used for the treatment of infertility unless it is caused by hypothyroidism.

For patients with diabetes: It is very important that you keep track of your blood or urine sugar levels as instructed by your doctor. Check with your doctor right away if you notice any changes in your sugar levels.

If you think you have become pregnant while using this medicine, tell your doctor right away. You may need a larger dose of levothyroxine while you are pregnant.

Women who are post-menopausal or who use this medicine for a long time may have some bone loss, which could lead to osteoporosis. Talk with your doctor if you have questions or concerns about this.

Call your doctor right away if you or your child has rapid or irregular heartbeats, chest pain, leg cramps, headaches, nervousness, irritability, sleeplessness, tremors, a change in appetite, weight gain or loss, vomiting, diarrhea, excessive sweating, heat intolerance, a fever, changes in menstrual periods, hives, or a skin rash. These could be symptoms of too much medicine in your body.

This medicine may cause severe hypothyroidism, called myxedema coma, which may be life-threatening and requires immediate medical attention. Check with your doctor right away if you have weakness, confusion or non-responsiveness, feeling cold, low body temperature, swelling of the body, especially the face, tongue, and lower legs, or difficulty breathing.

Do not suddenly Stop taking levothyroxine without first checking with your doctor. Your doctor may want you or your child to gradually reduce the amount you are using before stopping completely.

Make sure any doctor or dentist who treats you knows that you or your child are using this medicine. You or your child may need to stop using this medicine several days before having surgery or medical tests.

A temporary loss of hair may occur during the first few months of levothyroxine therapy. Ask your doctor about this if you have any concerns.

Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.

Serious side effects of levothyroxine

Along with its needed effects, levothyroxine may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking levothyroxine:

Get emergency help immediately if any of the following symptoms of overdose occur while taking levothyroxine:

Symptoms of overdose

• change in consciousness
• cold, clammy skin
• confusion
• disorientation
• fast or weak pulse
• lightheadedness
• loss of consciousness
• sudden headache
• sudden loss of coordination
• sudden slurring of speech

Other side effects of levothyroxine

Some side effects of levothyroxine may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects.

Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

For healthcare professionals

Applies to levothyroxine: compounding powder, injectable powder for injection, intravenous powder for injection, intravenous solution, oral capsule, oral solution, oral tablet.

General adverse events

The more commonly reported adverse events have included those of hyperthyroidism due to therapeutic overdose including arrhythmias, myocardial infarction, dyspnea, muscle spasm, headache, nervousness, irritability, insomnia, tremors, muscle weakness, increased appetite, weight loss, diarrhea, heat intolerance, menstrual irregularities, and skin rash.

Cardiovascular

• Frequency not reported: Palpitations, tachycardia, hypertension, arrhythmias, increased pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest^[Ref]

Cardiac function was evaluated in 20 patients requiring TSH suppression for either thyroid goiter or following thyroidectomy and radioactive iodine therapy for thyroid cancer and in 20 age and sex-matched controls. TSH suppression was associated with an increased incidence of premature ventricular beats, an increased left ventricular mass index, and enhanced left ventricular systolic function. The clinical significance of these changes remains to be determined.

Overtreatment with this drug may cause an increase in heart rate, cardiac wall thickness, and cardiac contractility and may precipitate angina or arrhythmias, particularly in patients with cardiovascular disease and in elderly patients.^[Ref]

Endocrine

• Frequency not reported: Changes in symptom presentation for diabetes and adrenal cortical insufficiency^[Ref]

Nervous system

• Frequency not reported: Headache, hyperactivity, insomnia, seizures, pseudotumor cerebri (children)^[Ref]

Dermatologic

• Frequency not reported: Hair loss, flushing, urticaria, pruritus, skin rash, angioedema, excessive sweating^[Ref]

Musculoskeletal

• Frequency not reported: Tremors, muscle weakness, muscle cramps, increased risk of osteoporosis, slipped capital femoral epiphysis (children)^[Ref]

A study evaluated the effect of long-term thyroid hormone therapy on bone mineral density in 196 women (mean age, 74.4 years) compared to a control group comprised of 795 women (mean age, 72.1 years). The mean daily thyroxine dose was 1.99 mcg/kg (range, 0.3 to 6.6 mcg/kg) with a mean duration of therapy of 20.4 years (range, less than 1 to 68 years). Women taking daily doses of 1.6 mcg/kg or more had significantly lower bone mineral density levels at the ultradistal radius, midshaft radius, hip, and lumbar spine compared to controls. However, estrogen use appeared to negate the adverse effects of thyroid hormone on bone mineral density.

Higher rates of femur fractures have been found in males (p=0.008) prescribed long-term thyroid hormone therapy as compared to controls in a case-control analysis of 23,183 patients, from the United Kingdom General Practice Research Database, prescribed thyroid hormone.

Overtreatment may result in craniosynostosis in infants and premature closure of the epiphyses in children with resultant compromised adult height.^[Ref]

Gastrointestinal

• Frequency not reported: Diarrhea, vomiting, abdominal cramps^[Ref]

Genitourinary

• Frequency not reported: Menstrual irregularities, impaired fertility^[Ref]

Hypersensitivity

• Frequency not reported: Serum sickness, hypersensitivity to inactive ingredients^[Ref]

Hypersensitivity reactions have occurred; however, it has been attributed to the inactive ingredients. These reactions have included urticaria, pruritus, skin rash, flushing, angioedema, various GI symptoms (abdominal pain, nausea, vomiting and diarrhea), fever, arthralgia, serum sickness and wheezing. Hypersensitivity to levothyroxine itself is not known to occur.^[Ref]

Metabolic

• Frequency not reported: Increased appetite, weight loss^[Ref]

Immunologic

• Frequency not reported: Autoimmune disorders (e.g., chronic autoimmune thyroiditis)^[Ref]

Other

• Frequency not reported: Fatigue, heat intolerance, fever^[Ref]

Psychiatric

• Frequency not reported: Nervousness, anxiety, irritability, emotional lability^[Ref]

See also:

Frequently asked questions

• Does levothyroxine cause weight gain or loss?
• What can I eat for breakfast after taking levothyroxine?
• Can I take other medications with levothyroxine?
• Does levothyroxine cause hair loss?
• How long after taking levothyroxine can you drink milk?
• How long does levothyroxine stay in your system?
• How soon after taking levothyroxine can I take omeprazole?
• What is the difference between Levoxyl and Synthroid?
• What is the difference between Unithroid and Synthroid?

Further information

Levothyroxine side effects can vary depending on the individual. Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Note: Medication side effects may be underreported. If you are experiencing side effects that are not listed, submit a report to the FDA by following this guide.

Medical Disclaimer