Thyroid stimulating hormone, free thyroxine and cognitive ability in old age: the Lothian Birth Cohort Study 1936.

The current study investigated the associations between thyroid stimulating hormone (TSH), free thyroxine (T(4)) and cognitive ability (general ability, memory and processing speed), in a large age homogenous sample (n=659) of generally healthy euthyroid older adults. Associations were considered both at baseline (mean age wave 1=69.5 years; SD=0.8 years) and approximately 3 years later (mean age wave 2=72.5 years; SD=0.7 years). Results indicated mean level decreases across waves in both TSH (t=10.99, p<0.001) and T(4) (t=34.55, p<0.001). There were no significant associations between TSH and T(4) with any of the cognitive variables at either wave. There was no suggestion of non-linear associations. The lack of associations supports suggestions that the effects of thyroid hormones on cognition may be restricted to older individuals above a given threshold, and/or those with levels of thyroid hormones within the range defining clinical thyroid disorder.