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which requires the administration of 600 micrograms orally to normal subjects, followed by measurement of thyroxine in the blood over 24 to 96 hours, from which the AUC and the Cmax are determined.  For many drugs, this may be very appropriate for determining pharmacologic bioequivalence, acting as a surrogate for therapeutic bioequivalence.

However, in the case of a hormone like thyroxine, pharmacologic bioequivalence only provides part of the story, since absorption is only one component.  The biological effect of the medication must also be assessed.

Another important distinguishing factor of L-T4 is the prolonged half-life of approximately one week.  Presently, measures of bioequivalence are done after an acute dose, thereby overlooking the time required for hormone equilibration in body tissues.  Additionally, one can question the comparability of bioequivalence from a superphysiological dose of L-T4 in a normal person with an intact thyroid versus a patient with reduced or even no endogenous thyroid hormone production.  The present technique does not allow discrimination between smaller, more appropriate doses of L-T4.

In summary, in the case of hormone therapy,

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