L-Thyroxine Side Effects

L-Thyroxine or levothyroxine is a synthetic form of a hormone made in the body by the thyroid gland. Hypothyroidism, meaning an underactive thyroid gland, is common. It is often treated with replacement of the thyroid hormone in pill form, usually in the form of L-thyroxine. Some brand names are Synthroid, Unithroid and Levoxyl. Like any drug, there are potential side effects. These tend to be more common and more severe with increased doses.

Cardiovascular Side Effects

The heart may be affected by an increased heart rate or an irregular heartbeat. Some people may feel these as palpitations. An extreme case could cause heart failure or cardiac arrest.

Central Nervous System Side Effects

Levothyroxine can cause nervousness or jitters and some people develop tremors or shakiness. It may also cause trouble sleeping, headache or difficulty tolerating heat.

Gastrointestinal Side Effects

Nausea, vomiting and diarrhea are possible side effects of any drug, including L-thyroxine.

Other Side Effects

Due to the effects on metabolism, L-thyroxine can cause increased sweating or weight loss. Irregular menstrual periods may occur. One possible long-term side effect, especially from higher doses, is decreased bone density. This could lead to osteoporosis.