What Dr Lowe says...

[Guest guest]

http://www.drlowe.com/QandA/askdrlowe/dxthyrd.htm

Question: I'm a 29-year old woman with fibromyalgia. My blood tests showed a normal T4 level and low T3 level. My doctor said that this lab result does not mean I have a thyroid-related problem. Do you agree?

Dr. Lowe: The answer depends on what we are referring to as a "thyroid-related problem." Most conventional thyroidologists use the word "thyroid" as a synonym for "thyroid gland." For the moment, let's accept this qualification for the sake of illustration and rephrase your question: Is there a thyroid gland problem that could result in a normal T4 level and a low T3 level? Theoretically, the thyroid gland may dysfunction in such a way that it secretes normal amounts of T4 but less-than-normal amounts of T3. This could result in a normal circulating T4 level and a low T3 level. T3 is the most metabolically active thyroid hormone, but a low circulating T3 level may not result in slowed metabolism and related symptoms. The reason is that most T3 inside cells, where the hormone drives metabolism, is derived from the conversion of T4 to T3. As long as enough T4 reaches the cells and the cells convert enough T4 to T3, metabolism may be normal despite the low circulating T3 level.

Using the term "thyroid" to refer to the thyroid gland, however, is an unfortunate convention. The absence of the qualifier "gland" can leave one confused as to what mechanisms are included in the term "thyroid-related problem." Some writers use this term to refer to problems related to the cellular processing or cellular action of thyroid hormones. If we use this meaning of the term "thyroid-related problem," there are several mechanisms that may cause a normal circulating level of T4 and a low level of T3. These mechanisms may or may not result in slowed metabolism and related symptoms.

For example, various man-made chemical contaminants may induce liver cells to selectively clear T3 from the circulation at a faster rate than they clear T4. Again, if enough T4 enters cells and is converted to T3, metabolism may remain normal despite the rapid clearance of T3 through the liver. Also, for various reasons, the enzymes that convert T4 to T3 (5'-deiodinases) inside cells may not be catalyzing the conversion at a normal rate. This may cause enough slowing of metabolism to result in symptoms, although there is some debate about this in the thyroidology literature.

These examples certainly don't exhaust the possible mechanisms that can result in this pattern of hormone blood levels. What is important to appreciate is that problems of either the thyroid gland or the cells of various tissues can produce normal T4 but low T3 levels. If you have symptoms of hypometabolism, and your lab result (a normal T4 and low T3) is consistent with repeated testing, further investigation is warranted to determine exactly what the mechanism might be. Since you have fibromyalgia, which means you have symptoms of hypometabolism, then further testing and investigation is certainly warranted in your case.

So, to simply answer your question: "My doctor said that this lab result does not mean I have a thyroid-related problem. Do you agree?" If the lab result is consistent with repeated testing, then I disagree. Your test result does not rule out a thyroid gland problem, and it does not rule out a possible problem in the cellular processing or action of thyroid hormones. Your symptoms are as important as your test results. Since your fibromyalgia symptoms indicate that you are hypometabolic, then some "thyroid-related problem" is a distinct possibility. Certainly, further testing is justified.