The quick and dirty summary from the 2nd link I posted above is probably the best 80/20 rule-approved paragraph (except for the pitch for dried urine testing :-)):
I tracked my rT3 from 26 to 6 ng/dL over years and felt exactly zero difference and exactly zero difference in my resting basal temperature.
The Bottom Line – Is the rT3 Test Useful?
There appears to be little clinical justification for routine testing of rT3 for thyroid function assessment [14]. The main applications of an rT3 test are as a prognostic/diagnostic indicator in non-thyroidal illness syndrome or to monitor patients receiving amiodarone, which inhibits liver type 1 deiodinase activity and increases rT3 relative to T3. A healthy balance of rT3 and T3 levels can be maintained and thyroid function kept optimal with proper nutrition – avoid extreme dieting or severe carbohydrate restriction which create a starvation signal; normal iodine and selenium levels (assessed with dried urine testing); and avoidance of excessive stress.
If you really want the full version:
https://mdedge-files-live.s3.us-east-2.amazonaws.com/files/s3fs-public/Document/May-2018/gomes-lima_reverset3orperverset3.pdf
After months of review and study, my usage of high levels of T3 (as part of combination therapy with T4) may have been main culprit of my atrial fibrillation (when T3 was combined with supra levels of test along with reasonable dosages of ND and oxandrolone). I have since moved back to T4 monotherapy and do not monitor my rT3. I will give @enackers credit on the diet recommendation. Keto long term is probably not wise as it induces nutritional starvation cascade to the body’s hormone levels. I have reintroduced smart carbs back into my diet with no adverse effect on my body fat level. Keto short term is a fantastic jump start on the 1-2 year process of dramatic body transformation.