what are the Common Levothyroxine Side Effects?

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  • Written By: Jessica Reed
  • Edited By: Heather Bailey
  • Last Modified Date: 04 November 2019
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Levothyroxine side effects range from common, mild side effects including changes in appetite and weight change, upset stomach, insomnia, headache, and temporary hair loss to more serious side effects that require medical attention including chest pain, irregular heartbeat, or an allergic reaction. The medicine is intended to create a more normally functioning thyroid in patients whose thyroid glands do not produce the required amount of thyroid hormones. Hair loss is common with levothyroxine as the body adjusts to the new thyroid hormone levels, but this symptoms should resolve itself after two or three months of treatment. If it does not or the patient finds this side effect particularly distressing, she can consult her doctor for advice.

Severe reactions include allergic reaction and problems including tightness or pain in the chest, difficulty breathing, or a rapidly beating heart. Allergic reaction symptoms may also include these symptoms along with a rash or swelling. If this occurs, the patient should talk to her doctor or go to the emergency room at the nearest hospital if the problem is severe or she has trouble breathing.


As the medicine begins to work in the body, the patient may notice a change in her appetite and weight. Stomach problems including nausea, diarrhea, cramps and vomiting may occur. If this persists or is troublesome, patients should contact a doctor. The patient may also have difficulty sleeping and notice an increase in anxiety. Headaches and tremors are also common physical side effects of levothyroxine.

Hair loss may bother the patient, but after the body adjusts to the new treatment her hair should grow back. The patient should speak with her doctor about any hair loss and he can advise her what to expect and when it should return. Many times the discomfort of mild levothyroxine side effects, particularly temporary effects that fade after one to three months, is outweighed by the necessity of treatment.

Troublesome side effects or those that interrupt daily living — such as a sensitivity to heat or constant headaches — may indicate that levothyroxine is not the best choice in medication. Levothyroxine side effects that cause changes in mood or dramatic changes in appetite may also indicate a problem with the medicine. Serious levothyroxine side effects are rare, but require immediate medical attention and typically mean the patient must stop taking levothyroxine all together.

If the patient becomes pregnant or must undergo a major event such as surgery, she should contact her doctor to find out whether to keep taking levothyroxine or to stop the medicine temporarily. Even if levothyroxine side effects are mild, the patient should still consult her doctor if they bother her. A different medicine may be the best approach to deal with hypothyroidism without interfering with the patient's lifestyle.


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