https://www.sciencedaily.com/releases/2 ... 084337.htm
RosieRemarkably, the CFS patients had lower serum levels of certain key thyroid hormones such as triiodothyronine (T3) and thyroxine (T4), but normal levels of thyroid-stimulating hormone.
These CFS patients, however, had relatively higher levels of another thyroid hormone called "reverse T3" or rT3. This appeared to be due to a shift in hormone production, where the body preferred to convert T4 to rT3 instead of producing T3. The low T3 levels found in CFS patients coupled with this switchover to rT3 could mean that T3 levels are severely reduced in tissue.