Many Internet sites and books falsely state that many people cannot convert T4 into T3. Converting T4 into T3 is necessary for life, and there is no evidence that this metabolic derangement can be acquired later in life. Anyone who could not convert T4 into T3 would not be alive, as this conversion is necessary for fetal development and birth.
This myth leads patients into potentially dangerous decisions about their treatment and into purchasing thyroid “support products” or T3 that they may not need. For information on how T3 levels are determined by measuring levels of the thyroid-stimulating hormone (TSH), see page 20.
The T4/T3 conversion myth is also used to support other false statements. One is that TSH tests can read as normal when you’re really hypothyroid (this is false except for rare people with pituitary tumors). Another falsity is that dessicated pig thyroid hormone (sold as “natural” thyroid hormone), questionable herbal concoctions that “help” with T4/T3 conversion, or regular and chronic T3 supplementation is preferable to, or healthier than, taking levothyroxine alone at the appropriate dose.
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The Truth About Reverse T3
Many websites and books say that trauma and stress can create a “fake” normal T3 level, called reverse T3. The claim is that many people with so-called normal TSH levels really have reverse T3 and are actually hypothyroid. This theory takes fact and creates fi ction.
T4 is converted to T3 by removing a specifi c iodine atom from each of the T4 molecules. If a different iodine is removed, reverse T3 is formed. Reverse T3 is inactive; it cannot do the job of T3 and therefore will not suppress TSH. It will not affect TSH levels at all. If T4 is converted to reverse T3 without making enough regular T3, your TSH levels will be high and you will be hypothyroid. If the TSH levels are normal, so are the levels of regular T3 inside of your cells.