Reverse T3

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In response to a message from the pituitary gland in the form of thyroid stimulating hormone (TSH), a healthy thyroid gland makes about 80% T4 (thyroxine), an inactive thyroid hormone, and about 20% T3 (triiodothyronine), the active thyroid hormone. Then, as the body needs it, inactive T4 is converted to active T3.
For the body to utilize T3 properly, certain conditions must exist, most notably adequate cortisol and adequate iron. When the right conditions do not exist, instead of T4 converting to T3, the body converts T4 to Reverse T3 (RT3), another inactive hormone. This will also happen if the body senses it is starving, such as from extreme dieting, or if there is serious illness. There are apparently many reasons the body converts T4 to RT3; having some RT3 is normal, but not to the exclusion of the active hormone T3. 
Once the thyroid receptors inside the cells are clogged with RT3, it appears to be very hard to clear it out, even when the original conditions that caused it are reversed. It does not matter whether the T4 to be converted is produced by one's own thyroid or comes in the form of thyroid medication. The same process applies. And the same process can go wrong.
The best way to break the cycle is to take T3-only thyroid hormone, bioidentical liothyronine sodium (brands U.S. Cytomel®; Mexico Cynomel®; U.K., Australia Tertroxin®). When one takes T3-only hormone, the pituitary no longer needs to stimulate production of T4. Production of TSH is suppressed and there is no T4 produced to convert to RT3. Total T4, FT4, and RT3 quickly disappear from blood serum. Once RT3 is cleared from the receptors, the body can properly utilize T3 hormone.
The amount of T3 one takes must be increased slowly, or titrated up. The process of clearing takes about 12 weeks. Some individuals clear suddenly with a rush of T3 into the receptors; others clear gradually and feel slowly better throughout the process. Still others are never certain when they clear, but RT3 is no longer a problem.
The basic conditions that caused the rise in RT3 must be corrected or it is likely to happen again.
Details of taking T3-only medication here