Synthroid is a prescription drug used to treat hypothyroidism and a type of thyroid cancer. Learn about the common, mild, and serious side effects it can cause. Synthroid is a thyroid medicine that replaces a hormone normally produced by your thyroid gland to regulate the body's energy and metabolism. Synthroid is a prescription medicine used to treat hypothyroidism (low thyroid hormone). Find patient medical information for Levothyroxine (Levoxyl, Synthroid, Tirosint,and others) on WebMD including its uses, side effects and safety, interactions, pictures, warnings, and user. Levothyroxine, also known as L-thyroxine, is a synthetic form of the thyroid hormone thyroxine (T 4). [4][7] It is used to treat thyroid hormone deficiency (hypothyroidism), including a severe form known as myxedema coma. [4]. It may also be used to treat and prevent certain types of thyroid tumors. [4]. It is not indicated for weight loss. [4]. Synthroid (levothyroxine) is a thyroid hormone used to treat low thyroid hormone levels (hypothyroidism) in both children and adults. It's also used to treat a certain type of thyroid cancer. Synthroid is a prescription medicine used to treat the symptoms of hypothyroidism and an enlarged thyroid gland (goiter). Learn about dosage, side effects, drug interactions, warnings, and more. SYNTHROID® (levothyroxine sodium) tablets, for oral use is a prescription, man-made thyroid hormone that is used to treat a condition called hypothyroidism in adults and children, including infants. Learn about Synthroid (levothyroxine) dosage, including starting doses, dosage charts for adults and children, adjustments, and how to take it safely. Levothyroxine is used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormone. This medicine is also used to help decrease the size of enlarged thyroid glands (also called a goiter). Synthroid is an inactive T4 that requires conversion to active T3 in your tissues. If you have inflammation, gut issues, genetics that limit conversion enzymes, or endocrine-disrupting chemical exposure, this conversion fails and you remain hypothyroid despite normal labs.