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LONG TERM USE OF T3 WILL ULTIMATELY DAMAGE YOUR THYROID
This notion was dispensed with in Mind & Muscle #10. Here is a brief recap of the relevant findings.
The first study that looked at thyroid function and recovery under the influence of exogenous thyroid hormone was undertaken by Greer (7). He looked at patients who were misdiagnosed as being hypothyroid and put on thyroid hormone replacement for as long as 30 years. When the medication was withdrawn, their thyroids quickly returned to normal.
Here is a remark about Greer's classic paper from a later author:
"In 1951, Greer reported the pattern of recovery of thyroid function after stopping suppressive treatment with thyroid hormone in euthyroid [normal] subjects based on sequential measurements of their thyroidal uptake of radioiodine. He observed that after withdrawal of exogenous thyroid therapy, thyroid function, in terms of radioiodine uptake, returned to normal in most subjects within two weeks. He further observed that thyroid function returned as rapidly in those subjects whose glands had been depressed by several years of thyroid medication as it did in those whose gland had been depressed for only a few days" (8)
These results have been subsequently verified in several studies (8,9) and a large number of trials where T3 was used to treat obesity. So, contrary to what has been stated in the bodybuilding literature, there is no evidence that long-term thyroid supplementation will somehow damage your thyroid gland.
(8) Vagenakis AG, Braverman LE, Azizi F, Portinay GI, Ingbar SH. N Engl J Med 1975 Oct 2;293(14):681-4
(9) Krugman LG, Hershman JM, Chopra IJ, Levine GA, Pekary E, Geffner DL, Chua Teco GN J Clin Endocrinol Metab 1975 Jul;41(1):70-80