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FOOD AND DRUG ADMINISTRATION - page 144 / 263

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dose is titrated to the proper level based on a combination of both laboratory and clinical parameters.  The former includes specifically the TSH level which is targeted to return to a relatively narrow normal range.

This is because, as you've heard, the effects of both under-treatment and over-treatment are potentially harmful.  Specifically, excess T4 replacement producing a low serum TSH, as reported by Sawin in the New England Journal of Medicine in 1994 and as reviewed by us in that journal in February 2001, can produce atrial fibrillation in as many as 30 percent of patients above the age of 60.

My review of the FDA guidance of bioequivalence of L-thyroxine sodium indicates that it is possible to consider two preparations bioequivalent, based upon T4 pharmacokinetics which fall between minus 80 to plus 125 percent of the reference compound.

As a physician who cares for many patients with hypothyroidism, I am concerned that the application of the existing guidelines for bioequivalence will yield results which do not properly reflect therapeutic equivalence.  It has been well documented that even with a normal blood level of T4, a low TSH level predicts

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