Medications that treat hypothyroidism contain thyroxine sodium, or L-thyroxine, as their active ingredient. Hypothyroidism occurs when the thyroid gland cannot produce enough hormone. Thyroxine sodium replaces one of the hormones the thyroid does not produce in adequate amounts. Medications such as Synthroid and Levoxyl contain thyroxine sodium. Side effects are rare but do occur in some people, according to the American Thyroid Association.
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Loss of Bone Strength
Pharmaceutical product guidelines for use of Synthroid and Levoxyl, both of which contain thyroxine sodium, state that use of the drugs for too long, or at too high a dose, may lead to a decrease in bone density, especially in postmenopausal women. Bone loss may lead to osteoporosis, a bone disease that causes an increased risk of bone fracture.
Patients may experience a racing or skipping heartbeat and shortness of breath, which may in turn lead to a feeling of increased anxiety, nervousness, shakiness or restlessness. According to the American Thyroid Association, side effects often occur because a dose is too large or too small. Too large a dose can cause many processes, including the heartbeat, to speed up.
Thyroxine sodium can increase a patient's metabolism, causing a patient to sweat more or feel hot when those around her do not. In the case of too small a dose, a patient may feel cold and have dry skin.
Diarrhea or Constipation
Thyroxine sodium can affect the gastrointestinal system. With a high dose, a patient may experience diarrhea as bodily processes speed up. In the case of a low dose, a patient may experience constipation and abdominal cramps.