Pilot study on the absolute and relative bioavailability of Synthroid and Levothroid, two brands of sodium levothyroxine.

The absolute and relative bioavailability of levothyroxine was determined in young, healthy male subjects after administration of i.v. solution, p.o. solution, or one of two brand-name, commercially available tablets. The mean dose size was about 0.18 mg. Serum concentration-time data were obtained over a period of 50 h. The absolute bioavailabilities of the two tablet preparations were 66% and 72%, respectively.