Levothyroxine (Synthroid, Levoxyl, Levothroid) is used by millions of people daily for treatment of hypothyroidism. It is a synthetic replacement for T4 thyroid hormone, which people with an underactive thyroid no longer make efficiently. Because levothyroxine is very similar to human thyroid hormone, it is usually well-tolerated, but some people do experience side effects.
Cardiac Effects From Levothyroxine
When the dose of levothyroxine is too high, patients often notice palpitations or a feeling that the heart is racing. This is particularly noticeable with exercise. Blood pressure can also elevate. Atrial fibrillation, an arrhythmia where the heart beats irregularly, can result, which increases the risk of stroke.
Patients on levothyroxine who are experiencing palpitations should notify their health care provider. A decrease in medication dose may be necessary.
Bone Loss
Patients whose levothyroxine dose is too high for too long can experience a decrease in bone mineral density leading to osteopenia or osteoporosis. This increases the risk of fractures, particularly in the spine or hips.
Female patients who are postmenopausal should have a bone density test to measure their bone mineral density.
Anxiety and Nervousness
Too much levothyroxine can be stimulating, similar to too much caffeine. Patients notice increased nervousness and irritability. This can also lead to sleeplessness due to racing thoughts and tremors.
Heat Intolerance and Sweating
Occasionally, too much thyroid hormone can trigger hot flashes, similar to those experienced in menopause. Patients may notice increased sweating and flushing.
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