Levothyroxine sodium tablets and injection contain synthetic crystalline L-3,3',5, 5'-tetraiodothyronine sodium salt [levothyroxine (T4) sodium]. Synthetic T4 is identical to that produced in the human thyroid gland.

Levothyroxine (T4) Sodium has an empirical formula of C15H10I4NNaO4xH2O, and molecular weight of 798.86 (anhydrous).

Synthroid Tablets Inactive Ingredients: Acacia, confectioner's sugar (contains cornstarch), lactose, magnesium stearate, povidone, and talc. The following are the color additives by tablet strength: 25 mcg: FD&C yellow No. 6; 50 mcg: None; 75 mcg: FD&C red No. 40, FD&C blue No. 2; 88 mcg: FD&C blue No. 1, FD&C yellow No. 6, D&C yellow No. 10; 100 mcg: D&C yellow No.10, FD&C yellow No. 6; 112 mcg: D&C red No. 27 & 30; 125 mcg: FD&C yellow No. 6, FD&C red No. 40, FD&C blue No. 1; 150 mcg: FD&C blue No. 2; 175 mcg: FD&C blue No. 1, D&C red No. 27 & 30; 200 mcg: FD&C red No. 40; 300 mcg: D&C yellow No. 10, FD&C yellow No. 6, FD&C blue No. 1.

Synthroid Injection Inactive Ingredients: 10 mg mannitol, USP, sodium hydroxide, 0.7 mg tribasic sodium phosphate, anhydrous dodecahydrate. Levothyroxine sodium powder for reconstitution for injection is a sterile preparation.