Study referenced as 238 says:
Serum ferritin (SF) was measured using immunoradiometric assay. Iron deficiency was defined as SF of <15 μg/L.
Study referenced as 239 says something but the abstract is very thin and I could not find the full paper.
Study referenced as 240 says:
Criteria for case inclusion were: a) absence of any systemic diseases, except for iron deficiency without anemia (hemo-globin >12mg/dl, serum ferritin <12μg/l and transferrin saturation <16%)
That paper is fascinating and suggests that rT3 levels are higher in those with low iron.
Study referenced as 241 is, again, not available in full so cannot assess.
Study referenced as 242 is available and interesting, but in a quick look, I could not even find the word "ferritin".
Very interesting material. The response of the pituitary to TRH is blunted in low iron. Hypothermia in iron-deficiency might be mediated by lowered T3 availability, ...
Unfortunately, in my view, still nothing which really bags the "ferritin must be 70" statement!
I suggest that somewhere in the region of 70 might be reasonable. But the tests for ferritin vary in just the same was as thyroid hormone tests - from lab to lab. So the actual numeric value could also be expected to vary.
I suggest that other factors such as inflammation, even vitamin C deficiency, need to be considered when interpreting any ferritin result - not simply the number on its own.
And I also suggest that even if 70 were optimum, it should be clearly recognised that there is a scale - and that 60 is better than 50, which is better than 40, ... There is not a sudden 70 is good; 69 is bad.
Finally, ferritin is an iron storage protein. It takes time with improved iron for ferritin levels to rise. At some points it might be possible to have entirely adequate non-ferritin iron levels, with the ferritin levels lagging behind.
(I have always felt ignorant of how iron "works". If anyone reading the above feels I have made a big mistake, please let me know.)