Reverse T3, like cholesterol, is a natural substance found in every single body, and has a purpose. To try and rid oneself of it is unnatural, and analogous to taking statins to bring cholesterol levels down. Many have suffered permanent damage from taking statins, and people have suffered serious side effects from taking T3-only. The rule that the reverse T3 ratio should be greater than 20 is analogous to saying one’s TSH should be a certain number. Both are arbitrary numbers that fluctuate and should not dictate treatment. [TSH levels do not reflect thyroid levels]
Below are three different lab profiles where the reverse T3 ratio can be low (in patients already taking thyroid medication). Each has a different cause and requires a different treatment, which is not accounted for with the simplistic reverse T3 ratio formula, which is: if FT3 divided by rT3 is less than 20, then one should only take T3. This is called nonsense math, where an equation gives an appearance of credibility, even though it makes no sense.  If a low ratio can be from both high or low Free T3, then the ratio is useless, because it can mean two different things; this makes it mathematically invalid. BMI or Body Mass Index is an example of a valid ratio. Weight is divided by height. The higher the BMI ratio, the more overweight the person. A high BMI ratio does not mean overweight in some cases, but underweight in others. But a low rT3 ratio can mean both too much or too little T3, as explained below.
Profile I: mid-range to high Free T3, below mid-range Free T4
Reverse T3 may be slightly high (a few points above the reference range) due to high T3. This lab profile is often found in those taking desiccated thyroid, because of the high T3/T4 ratio in the pills. The body senses the high T3 levels, and converts some T4 to rT3 since additional T3 is not needed. Desiccated thyroid also contains rT3, so reverse T3 levels should rise as the desiccated dose is increased. Lowering the desiccated thyroid dose and adding T4 has lowered rT3 levels in some patients. These people report improved reverse T3 ratios on higher levels of T4 and say they feel more “balanced” with the additional T4. The T3-only protocol does not work well for these patients because it compounds the problem that caused the high reverse T3 in the first place—too much T3. These people feel best with a higher proportion of T4 to T3, and may suffer severe side effects on the T3-only protocol. The rT3 ratio is too simplistic to differentiate these people from those in Profile III.
Profile II: below mid-range Free T3, mid-range to high Free T4
Reverse T3 may be high due to poor conversion and lack of essential cofactors like selenium. This lab profile is often found in those who only take T4. Reverse T3 levels may actually be within the reference range, but the calculated rT3 ratio is low because T3 levels are low. Changing to desiccated, which has T3, or adding T3 to a lower dose of T4 often helps. Addressing factors that impede conversion, such as high cortisol or low iron, is also recommended. These people do best when their T4/T3 ratio is closest to that found in desiccated thyroid. This ratio can also be attained with synthetic T3 and T4, or customized by adding additional T3 to the desiccated thyroid dose. These people feel better once they add some T3 to their dose.
Profile III: below mid-range Free T3, below mid-range Free T4, over range reverse T3
If reverse T3 is significantly over the reference range (sometimes hundreds of points over the range), and the patient’s Free T3 and Free T4 do not rise with increasing doses of any type of thyroid hormone, this could indicate something more serious that should be investigated. Iron labs may point to Anemia of Chronic Disease, rather than Iron Deficiency Anemia. Serious infections (root canals, for example) can cause this. Uterine fibroid or other tumors, as well as damaged heart muscle could also cause high reverse T3. The high reverse T3 is a sign of a problem, not the cause of the problem. Reverse T3 itself does not physically block the receptor. Surgical removal of the infected part has often brought a relief in symptoms. These are the patients that must take extremely high doses of T3 to overcome an overactive D3 enzyme, which not only converts T4 to reverse T3, but also inactivates any T3 to T2, resulting in minimal T3 for the body’s functions. These people do best with a higher proportion of T3 to T4, but may have a hard time tolerating it due to their iron problems. Some of these patients do not tolerate any T4 because of their underlying condition and only feel functional on T3-only. However, they are not immune from the side effects of too much T3, and their T4 deficiency may affect other parts of the body that depend on T4, especially the brain.
Healthy Thyroid Levels
Many seem to feel well when both their Free T3 and Free T4 are above mid-range. As shown by the graphs above, this is difficult to achieve with only one type of medication. For me, lowering my desiccated thyroid dose and adding in T4 dropped my rT3 back to mid-range, and brought both FT3 and FT4 into the upper half of the reference range.
70. Eriksson, Kimmo. The nonsense math effect. Judgment and decision making 7 (2012): 746-749. http://www.sas.upenn.edu/~baron/journal/12/12810/jdm12810.pdf