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Only T4 is Transported into the Brain A Widespread False Belief

Not a month passes that someone, a doctor or a patient, writes or says to me, “T3 cant pass the blood-brain barrier into the brain.” These people include this phrase in a variety of contexts. Some ask questions, others make assertions, and some criticize a statement of mine thats relevant to this belief. But each communication is undergirded by the presumption that the belief is true.

To illustrate, two days ago, a patient emailed this typical statement to me: “I’ve been doing more reading on the Internet. An article I came across talked about T3 not being able to cross the blood-brain barrier, and for that reason, it’s good to take some T4 as well as T3.”

Some patients and physicians include the presumption in thoughtful discourse. For example, two weeks ago, a physician wrote and asked essentially: “Since T3 does not cross the blood-brain barrier, then the T3 a patient takes orally cannot influence the pituitary glands secretion of TSH. But this doesnt reconcile with myclinical experience. Ive seen scores of patients TSH levels decrease in response to oral T3 medication. I guess thyroid physiology is just very complex.”

Let me say right up front, resoundingly, that this belief is false! It is an institutionalized falsehood that tenaciously thrives in the minds of both patients and physicians. It reminds me of gossip I used to hear as a young man in small backwood towns of South Alabama and Northwest Florida. Someone would say, “Word is, ol Mathews been cheatin people round town. Must be so, cause they dont let him pay with checks at the grocery store anymore.” I was amazed at how stubbornly these people held to their allegations, despite evidence that they were ill-founded. If they stood in line behind ol Mathews at the grocery checkout counter and watched him pay with a check, they would later tell others how stupid the clerk was for forgetting to reject the check from the ol cheat. Then, undaunted, they would go right on gossiping about ol Mathews.

I earnestly hope that patients and physicians are more corrigible than those goodol country folks were. We must dispel this false belief because it can influence decisions about patients thyroid hormone therapy. Ive had patients through the years who had fully recovered using T3 alone with no adverse effects whatever, only to become ill again when a well-intended doctor reduced the patients T3 dose and added some T4just to make sure the patients brain wasnt deprived of thyroid hormone.

Five years ago, on the same day, I received letters about this belief from a medical doctor and chiropractic doctor, both of whom practiced in France. As I read their letters in close tandem, it brought to mind the trail of similar letters to me that spanned back, at the time, some twelve years. I reacted by researching and writing an article in which I showed that the belief is false. At the time, I was deluged with other responsibilities, and so I didnt post the article to or submit it elsewhere for publication. Each time I received another letter or email from a patient orphysician, I thought again about publishing the article.Finally, on August 17, 2005, we published the article on Before doingso, I revised it to include a new discovery about T3 and the brain. Since Ive continued to receive correspondence in which authors reiterate the false belief, I decided to write this piece to bring attention again to the 2005 article.

This false belief is virtually an indelible stain in the memory of patients and physicians, and to clear it away, I think well have to scrub long and loud with thesolvent of truth. The new electronic journal Thyroid Science(, a publication for patients, clinicians, and researchers, will become active later this year. When it does, we must publish a review paperdocumenting that T3 does indeed enter the brain. And we must keep the paper prominently display for the world to see.

For those who dont want to read the 2005 article, Ill summarize here what I documented in the article: “Transthyretin” is the protein that transports T4 acrossthe blood-brain barrier. It also transports T3 across. It may transport less, however, because the binding affinity of the receptors within the protein is about ten times less for T3 than for T4. Nonetheless, this protein does indeed transport T3 and T2 into the brain across the blood-brain barrier. However, T3 also enters the brain, at least in mice, without riding in on transthyretin.

Transthyretin also transports chemical contaminants such as dioxins and PCBs across the barrier. The latter chemicals displace T4 and T3 from the thyroid hormone receptors inside the transthyretin molecule. Since all humans are polluted with these contaminants (this is what toxicologists tell me), the displacement of T4 and T3 may be a major mechanism of health problems from abrain deficiency of thyroid hormone.

The potential health problems from this displacement are highly complex. In The Metabolic Treatment of Fibromyalgia, I wrote an extensive section on the topic. In the section, I noted that after contaminants displace T4 and T3 from transthyretin, bind to the hormone receptors in the protein, and then ride the protein into the brain, the contaminants can bind to thyroid hormone receptors on genes. The binding altersthe normal transcription activities of the genes, producing adverse effects that are hard to predict and diagnose.

To sum up, the belief that T3 doesnt cross the blood-brain barrier is false. The documentation is far from new. It is high time, then, that we pluck this false belief out of patients and doctors frames of reference and dispose of it once and for all.


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