After seeing this in another discussion I though I'd do a little bit of web surfing.
I think we need to take into account a few things before we all decide to reduce our levo/T3/NDT.
1) Studies on calorie restriction with mice, rats, worms etc. They had no choice. Yes it might have reduced their insulin and other aging biomarkers, but how bad tempered were they? How did they feel? Apparently many showed signs of clinical depression by the end of the study. So that's a NO (unless for a short time).
2) Studies on humans. Mostly Leiden experiment and similar where families who had many members reaching their 90s were tested and found to have a genetic predisposition to low thyroid and higher TSH. The reports imply that these people all felt fine (so there's no way they'd have had a full thyroid panel if not for the study). It seems that there is a "phenotype" with "subclinical hypo". I mean really subclinical - no symptoms. I mean, if you feel fine, why would you get a test? The researchers point out that it is difficult to tell the difference between low thyroid results from diseases of aging and healthly people with low thyroid results.
Does that explain why our references ranges are high enough to make many of us feel ill? Because there a bunch of healthy people with that phenotype messing up the results for the rest of us?
If I reach 90 and have a TSH of 4 and low normal T4 and T3, while feeling great, I'll thank my Dutch Jewish granny for her genes. Since I appear to have secondary hypo, I don't think It's going to happen.