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Lower-But-Normal   Serum TSH level Is Associated With the Development or Progression of Cognitive   Impairment in Elderly: Korean Longitudinal Study on Health and Aging

Moon   JH1, Park   YJ, Kim   TH, Han   JW, Choi   SH, Lim   S, Park   do J, Kim   KW, Jang   HC.
J Clin Endocrinol Metab. 2014 Feb;99(2):424-32 PMID: 24285689
The   association between subclinical hyperthyroidism and the risk of dementia has   been validated in several studies. However, the effect of thyroid function   within reference range on the risk of cognitive dysfunction including mild   cognitive impairment (MCI) and dementia is still unclear.: We conducted a   population-based prospective study as a part of the Korean Longitudinal Study   on Health and Aging. A total of 313 participants who were euthyroid and non-demented   at baseline and completed cognitive function tests at a 5-year follow-up   evaluation (mean age 72.5 ± 6.9 y) were analyzed in the present study.:   Baseline thyroid function was compared according to the development of MCI or   dementia during the study period. Binary logistic regression analysis was   performed to investigate the independent association between thyroid function   and cognitive impairment.
At baseline evaluation, 237 subjects   were cognitively normal, and 76 subjects had MCI. Diagnoses of cognitive   function in 259 subjects remained unchanged or improved during the study   period (non-progression group), whereas 54 subjects showed progression of   cognitive impairment to MCI or dementia (progression group). In the   progression group, baseline serum TSH levels were lower than those in   nonprogression group. Baseline serum free T4 levels were not significantly   different between these two groups. The association between lower baseline   serum TSH levels and the development of MCI or dementia was maintained after   adjustment for conventional baseline risk factors.
Lower serum TSH level within the reference range was independently associated   with the risk of cognitive impairment including MCI and dementia in elderly   subjects.

 Comment-  Maybe a chicken and/or egg question. SC  hyperthyroidism,  that is, low TSH with   normal range FT4, and now low TSH within the normal range, are associated   with developing dementia. Or is it the other way around? It is very possibly that   dementia represents one fiorm of the Non-Thyroidal Illness Syndrome,  and via altered hypothalamic function, leads   to lower TSH levels. This interpretation fits with the observed low TSH levels, but normal thyroid hormone levels.