Thirty-two patients with primary hypothyroidism were given oral thyroxine as Levothroid or Synthroid to see if the two preparations had similar effects. The serum thyroxine was used as an index of bioavailability and the serum thyrotrophin as an index of biologic activity. The serum thyroxine was lower in all 32 patients when taking Synthroid than when taking Levothroid. In 15 patients the serum thyroxine level fell low enough to raise the serum thyrotrophin; in all 15 the serum thyrotrophin rose when taking Synthroid. Direct measurement of thyroxine in the tablets showed that the tablets of Synthroid contained 20% to 30% less thyroxine than their stated content. Thus, the decreased bioavailability (lower serum thyroxine) and decreased biologic action (higher serum thyrotrophin) of Synthroid were due to the lower content of thyroxine. An incidental observation is that the range of serum thyroxine in treated hypothyroid patients is 7.6 to 16.6 micrograms/dL, higher than in normal persons. Because oral thyroxine is widely used, a cooperative effort among manufacturers, the United States Pharmacopeia and Food and Drug Administration, and clinicians to ensure the potency and biologic action of oral thyroxine is in order. Meanwhile, it seems reasonable to use oral thyroxine that is close to the stated content.