Thyroid stimulating hormone and prospective memory functioning in old age.

Alterations of thyroid functioning are common in old age. Even among persons free from thyroid disorders, subclinical variations in thyroid functioning may affect cognitive performance. However, it is unknown whether prospective memory (ProM) is related to thyroid stimulating hormone (TSH) variations. An association could be expected, as changes in the thyroid gland have been linked to alterations in frontal brain regions that play a key role in prospective remembering. Thus, the aim of this study was to examine whether subclinical variations in thyroid functioning affect ProM performance. We studied 103 participants, 75 years and older, who were free from thyroid disorders and had serum levels of TSH and thyroxine (T4) within normal ranges. Interestingly, we found a non-linear association between TSH and ProM performance, where persons with TSH levels above the fourth quartile performed substantially better than persons in the other quartiles. T4 levels were unrelated to ProM performance. This pattern suggests that the previously identified "normal-range" interval for TSH should be moved further up in old age, at least when cognitive functioning is considered.