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FOOD AND DRUG ADMINISTRATION - page 193 / 263

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T4, as you've heard, is the synthetic version of the naturally occurring thyroid hormone.  There is no substitute for thyroxine.  All our patients require lifelong therapy and the medical community relies on thyroxine as being truly bioequivalent.

The decision of the committee here today is extremely important because 13 million Americans rely on thyroxine.

You've heard a little bit about TSH this morning.  I'd like to review it some more.  Here is the pituitary gland that makes and releases TSH, appropriately in the center of the slide.  It stimulates the thyroid gland to release T4 and T3 which circulate in the blood, binding to tissue receptor sites where the metabolic action of thyroid hormone is exerted.  There's negative feedback back to the pituitary and the hypothalamus turning off TSH.  So because we cannot look at all of these other tissue levels effectively, TSH is our window into the body where we can judge the effectiveness of a given level of T4 or a given dose of levothyroxine and its physiologic effects.

So we physicians use the TSH level to

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