Euthyrox is a medication that contains levothyroxine sodium, a synthetic version of the hormone thyroxine. It is used to treat hypothyroidism (an underactive thyroid gland), a condition in which the body does not produce enough of thyroid hormone. This medication is available in tablet form in strengths of 25 mcg and 50 mcg. It is typically taken once a day, with or without food. Common side effects of this medication may include headaches, insomnia, and weight gain.