There are rare cases when thyroid hormone doesn't work. In this case, you would have high TSH levels in spite of taking thyroid hormone, but this has nothing to do with T4/T3 conversion. Clearly, when you don't have enough T4 in your body, or it is being poorly absorbed from your pills, you won't make sufficient amounts of T3 either. Here are the common reasons why thyroid hormone doesn't work:
Thyroid Hormone Resistance (a.k.a. Refetoff Syndrome)
Thyroid hormone resistance is a rare genetic disorder in which the receptors that stick to T3 (then stick to specific parts of the chromosomes of each body cell) are mutated so that they don't stick to T3 very well. Because of this, much, much more T4 is needed to enter each cell and change to T3 to make up for this abnormal T3 receptor. In this case, you would have either high TSH levels, despite normal or high levels of free T4 or free T3 (making you hypothyroid), or normal TSH levels and very high levels of T4 and T3 (making you euthyroid because the high levels compensate for the resistance). In either case, just as in normal individuals, a high TSH reflects too little thyroid hormone and a normal TSH reflects sufficient thyroid hormone. We devote a separate chapter to thyroid hormone resistance (Chapter 18). If you have thyroid hormone resistance, a highly trained thyroid specialist needs to be on the case; as of 1997, only about 350 cases were seen worldwide! That means most endocrinologists have never seen a case. (Some material on the Internet refers to a nonexistent disorder called "thy-roxine resistance." This appears to be a distorted and misunderstood interpretation of the literature on thyroid hormone resistance. We deal with this and other distortions of thyroid hormone resistance in Chapter 19.)
Many people take thyroid hormone with too much food or with hot liquids, take it at irregular times on successive days, or just skip pills. All of these things can dramatically affect proper absorption of thyroid hormone or its efficacy. In these cases, you would have high TSH, and low levels of free T4 and free T3. See Chapter 10 on thyroid hormone, which outlines the correct way to take T4 for maximum efficacy and optimum absorption.
Taking Certain Medications or Supplements That Interfere with Absorption
If you take multivitamin pills or iron supplements such as ferrous sulphate, take your thyroid hormone pill at least five hours in advance or take the multivitamin at bedtime and your thyroid hormone tablet in the morning. Iron appears to bind to thyroid hormone, thus making less of it available for absorption into your body. In this case you would have high TSH levels. This is discussed more in Chapter 10.
Thyroid pills become inactive when they are exposed to heat. Use the "chocolate bar rule"; if you wouldn't store a chocolate bar in a given place for fear of it melting, don't store your thyroid hormone pills there, either.
Very rarely, certain malabsorption problems such as celiac disease can interfere with thyroid hormone absorption. In this case you would have high TSH levels.
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