What is levothyroxine and how should it be taken?

Levothyroxine is a synthetic form of the thyroid hormone T4 used to treat hypothyroidism and thyroid hormone replacement after thyroidectomy. It is the most prescribed medication in the United States. Levothyroxine should be taken on an empty stomach, 30 to 60 minutes before breakfast, with a full glass of water. Many substances reduce its absorption, including calcium supplements, iron supplements, antacids, and proton pump inhibitors, which should be taken at least four hours apart. Thyroid-stimulating hormone, or TSH, levels are monitored every six to eight weeks after dose changes.