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Synthroid vs. generic T4

Date: Wed, 29 May 1996 08:27:20 -0400 (EDT)
From: "Paul Luisada, M.D." <pvl2@cornell.edu>
Subject: Synthroid vs. generic T4

The preference for the Synthroid brand of l-thyroxine comes from some studies published 8 or 10 years ago (in the endocrine or internal medicine literature, I recall) showing higher T4 levels in patients when they were on Synthroid compared with a variety of generics at equipotent doses. The issue wasn't so much the demonstrated lower potency of the generics as it was the variability of levels between them.

The reasoning was that patients prescribed generic T4 end up on a variety of brands since the brand they get depends upon what's in stock at the pharmacy at the time. Fluctuating T4 levels result in the need to do more thyroid function tests. This wipes out several years' worth of savings from buying generic. The conclusion was that it is better to prescribe Synthroid for reasons of consistency.

These studies were done in patients receiving T4 for replacement purposes, not for antidepressant augmentation. At that time most practicing clinicians were watching T4 levels for the purpose of adjusting dosages in patients receiving replacement T4. Now the trend is to watch TSH, with the result that far fewer people are getting doses of T4 high enough to make the exogenous T4 dose the only factor determining their T4 levels.

Despite the lack of studies, a similar argument could be made in the case of T4 used for antidepressant augmentation. Here the dose is determined empirically by the patient's response, and in theory varying T4 levels might lead to fluctuating response levels.

I am not aware of similar studies with T3 replacement preparations. I suspect the frequency with which Cytomel is mentioned stems from the fact that it's the best-known or only brand of T3.

Date: Wed, 29 May 1996 08:05:26 -0700 (PDT)
From: Thomas Lewis <tblewis@itsa.ucsf.edu>
Subject: Synthroid vs. generic T4

You may have seen the item in the news about the researcher at UCSF who re-did the the studies described above and found no difference in the bioavailability of Synthroid and generics -- unfortunately, the study was industry-funded and she withdrew her paper from publication after the drug company invoked a secrecy clause in the contract...

Date: Wed, 29 May 1996 22:48:28 -0500
From: Stephen R Saklad <Saklad@uthscsa.edu>
Subject: Synthroid vs. generic T4

There is a brief review of this in Hospital Formulary or P&T (this month?). The study was funded by Boots, who then merged with Knoll. Knoll makes Synthroid.

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