Levothyroxine Side Effects

levothyroxine Side Effects

Levothyroxine is a synthetic thyroid hormone. It can be prescribed for patients suffering from hypothyroidism (low level of thyroid hormone). This drug can also be used to treat congenital hypothyroidism (cretinism), certain types of thyroid cancers, and can be used to prevent or decrease goiter (enlargement of the thyroid gland). Levothyroxine should not be used for other purposes (e.g. for treatment of obesity or infertility) if they are not caused by low thyroid hormone levels)

Side effects

Levothyroxine is usually well tolerated by the body and patients can take it for years without any side effect. However, some people may experience side effects more often due to overdose.

Hyperthyroidism (excessive thyroid hormone production) is the most common side effect of Levothyroxine. Symptoms of such medically induced hyperthyroidism include:

  • Tachycardia (high heart rate)
  • Mood changes
  • Hyperactivity
  • Nervousness
  • Irritability
  • Anxiety
  • Tremor
  • Feeling hot
  • Increased sweating
  • Flushing
  • Fever
  • An increased appetite, along with weight loss
  • Muscle weakness
  • Vomiting
  • Diarrhea
  • Stomach cramps
  • Difficult or painful breathing
  • High blood pressure
  • Heart palpitations
  • Chest pain
  • Insomnia
  • Hair loss
  • Menstrual problems
  • Seizures

If you experience any of these symptoms, contact your health provider to determine its source and adjust the dose of the medicine.

Other side effects of Levothyroxine are:

  • Fatigue
  • Severe or persistent headache
  • Heat intolerance
  • Excessive sweating
  • Changes in appetite (increased appetite or anorexia)
  • Nausea
  • Vomiting
  • Stomach cramps
  • Diarrhea (sometimes severe)
  • Fever
  • Joint pain
  • Lowing bone mineral density (especially in women)
  • Leg cramps
  • Muscle weakness
  • Seizures
  • Tremors
  • Shortness of breath
  • Chest pain
  • Arrhythmia (fast or irregular heartbeat)
  • Increased left ventricular mass index
  • Enhanced left ventricular systolic function
  • Trouble sleeping
  • Anxiety
  • Irritability
  • Nervousness
  • Unusual weight gain
  • Weight loss
  • Changes in menstrual periods

Like any other medicine, Levothyroxine can cause allergic reactions such as:

  • Dizziness
  • Breathing difficulty
  • Wheezing (unusual hoarseness)
  • Flushing
  • An unexplained rash
  • Urticaria
  • Hives
  • Itching
  • Unexplained swelling (especially of the face, lips, mouth, tongue or throat and neck)
  • Angioedema
  • Serum sickness

Serious side effects

Serious adverse effects usually develop due to hyperthyroidism. They can include arrhythmia (premature ventricular beats), heart failure, heart attack, heart arrest, osteoporosis, broken bones (femur fractures in males) or other signs of weakened bones (due to decreased bone density), hypoglycaemia, diabetes mellitus, fertility problems, myxedema coma, adrenal cortical insufficiency (especially without treatment of coexisting adrenal insufficiency).

Levothyroxine intake in elderly patients can increase the risk of coronary artery disease development.

Changes in laboratory tests

If patients who were treated with Levothyroxine have hyperthyroidism symptoms, their blood will show high level of fT4 (free tetraiodothyronine) and low level of TSH (thyroid-stimulating hormone).

Biochemical investigation results can show elevations in liver function tests (alanine aminotransferase, aspartate aminotransferase, lactat dehydrogenase, gammaglutamyl transferase, bilirubin, alkaline phosphatase).

Signs of osteopenia can also be found.

Side effects in pregnant women

During pregnancy, women (especially with known hypothyroidism) should closely monitor their levels of thyroid hormones, because low thyroid hormone levels  can be affect the pregnancy course and have negative effect on the baby.

In adequate doses, Levothyroxine is not harmful to the fetus.

However, if you become pregnant during Levothyroxine intake, you should contact your doctor to adjust the dose (usually to increase it).

Side effects in children

Overdose in infants can lead to craniosynostosis and premature closure of the epiphyses (it can lead to decrease in the future height of an adult person)

Cases of pseudotumor cerebri and slipped capital femoral epiphyses were reported in children who received Levothyroxine for hypothyroidism treatment.


You may never experience any of the side effects listed above. Your doctor cannot predict your body’s reaction to Levothyroxine until you have tried it. That is why you should immediately inform your doctor of any changes in your health condition after taking Levothyroxine. Therefore, when you feel something is wrong, ask your doctor for advice and he or she will diagnose if your problem is connected with the medicine intake. You should also be careful because some of the side effects of Levothyroxine require urgent medical attention.