Levothyroxine (Brand name: Synthroid): This thyroid replacement hormone is used to treat hypothyroidism. Left untreated, hypothyroidism can cause obesity, infertility, joint pain, and heart disease.
Levothyroxine is a synthetic version of a hormone called thyroxine. It replaces thyroxine if your thyroid gland cannot produce it and prevents the symptoms of hypothyroidism. Levothyroxine starts working straight away, but it may be several weeks before your symptoms start to improve.