Synthroid (generic: levothyroxine) is a prescription medication used to treat underactive thyroid (hypothyroidism), goiters, and thyroid cancer. It is a synthetic hormone used to replace hormone normally produced by the thyroid to regulate energy and metabolism.

This medication comes in tablet form to be taken once daily, in the morning, on an empty stomach.

Common side effects of Synthroid include rapid heartbeat, difficulty sleeping, nervousness, and headaches.