Twenty-eight patients with hypothyroidism or goiter were treated double-blind with replacement dosages of desiccated thyroid USP, thyroglobulin, liothyronine, and sodium levothyroxine. Laboratory profiles and clinical responses demonstrated that all compounds had a normalizing effect on the patients' metabolism. Bound T4(as represented by PBI concentrations) reflects serum free thyroxine levels, confirming a previous hypothesis that free and bound T4exist in an equilibrium conforming to a mass action equation. In contrast to what has previously been reported, the resin T3test was in the normal range for patients on liothyronine. There was a higher incidence of hyperthyroid symptoms on liothyronine therapy. When patients were treated with sodium levothyroxine, free T4and bound T4were high, at levels near those associated with clinical hyperthyroidism in untreated subjects.