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what a silly study!  They look at free T4, the weakest thyroid hormone and found high levels to be associated with dementia.  However, they failed to assess free T3, the most biologically active, and reverse T3, inactive.  This study was like they intentionally blinded themselves of the body's physiology to study dementia.  This is the problem with a lot of research today: they have forgotten their foundation. Many older adults struggle with poor free T4 to free T3 conversion and high reverse T3 conversion.  So, without knowing what these clients were doing in these pathways makes their conclusion obsolete.

Metformin shown to suppress TSH secretion without change in free t4 or free t3 levels.  This again, shows the lack of reliability of the test TSH.  With the suppression of TSH indicated in this study, the free t4 and t3 should have been elevated, but were obviously not.

low free T3:elevated reverse T3 correlates with major depression and TSH and free T4 do not.

T4 therapy (synthroid) alone does not increase serum free T3 concentrations in many patients

reverse T3 comes from peripheral conversion from free T4

low dose pesticide (organophosphates) exposure associated with delayed sexual development in boys and girls.  In addition, there was an inverse correlation with free T4 and a positive correlation with TSH.  

Here is a great review of thyroid hormone metabolism published in the very relevant journal Endocrinology.  The review describes the peripheral metabolism of the pro hormone T4 and the peripheral metabolism to free T3 and/or reverse T3 and the associated metabolic implications.  Why don't more endocrinologists read?This is an animal study.

Study found no significant effect of subclinical hypothyroidism on weight in the elderly.  However, higher free T4 levels were associated with lower baseline weight and weight loss in women. TSH and free T3 were not. associated.

reverse T3 does not block T4 to T3 converson; but occupies T3 receptors and inhibits T3 stimulating receptors

low free T3: elevated reverse T3 in critically ill patients

poor nutrition leads to low free T3 in tissue which lowers cellular metabolism which leads to obesity

This article finds the mean TSH level in the "healthy" population to be 1.5.  Calls into question any level above that with symptoms of hypothyroid.  Full, adequate thyroid including free T3 and rT3 must be done.