Can the metabolism be switched on and off?  When we switch off a light the light goes off and stays off until we switch it on again.  Recent research suggests that important enzymes that control the metabolism are set up in such a way that the metabolism can slow down and stay down until something is done so that it speeds up and stays up.  These enzymes are called deiodinase enzymes and they are important in converting T4 to Reverse T3 (RT3) and T3.  The enzyme that converts T4 to T3 is D2 (5′-deiodinase) and the enzyme that converts T4 to RT3 is called D3 (5-deiodinase).  When the activity of D2 in the cells is more prominent then the metabolism is “on” and when the activity of D3 is more prominent then the metabolism is “off”, since T3 is the active hormone and RT3 is inactive.

Here’s the interesting part, D3 makes more Reverse T3 and RT3 reduces D2.  Thus, when the metabolism is “off” it tends to stay off.  On the other hand, D2 makes more T3, and with less RT3 being produced, less D2 is destroyed.  Thus, when the metabolism is “on” it tends to stay on.  Giving patients T3 therapy reduces their Reverse T3 levels and reduces the destruction of D2.  The T3 can speed up the metabolism and removing the RT3 can help the body continue making T3 on its own.  It appears that T3 can be used, in many cases, to turn the metabolism “on” so it tends to stay on.

In fact, anything a person can do to improve the conversion of T4 to T3 can increase his/her chances of the metabolism turning on and staying on.

This research is especially interesting to me because I’ve seen evidence of this in patients and wrote about it in my books over 20 years ago. I hypothesized about these things because that’s what appeared to be happening in the patients.  It’s gratifying that the research is bearing it out.

Best,
Denis Wilson, MD

 

Gereben B, Zavacki A, Ribich S, Cellular and molecular basis of deiodinase-regulated thyroid hormone signaling, Endocrine Reviews Dec 1, 2008 vol. 29 no. 7 898-938