Changes in Serum Thyroid Function Predict Cognitive Decline in the Very Old: Longitudinal Findings from the Newcastle 85+ Study

Background: Low serum thyrotropin (TSH) has been associated with an increased risk of cognitive impairment in observational studies of older individuals, but the mechanism underlying this is unclear. We investigated the association between changes in thyroid status and cognitive impairment in very old adults, using prospective data from the Newcastle 85+ study. Method: A cohort of 85-year-old individuals was assessed for health status and thyroid function. Complete data from a comprehensive multidimensional measure of health and repeat thyroid function were available for 642 participants with normal free thyroid hormones and TSH levels ranging between 0.1 and 10 mU/L. Cognitive performance, assessed using Mini-Mental State Examination (MMSE) and Cognitive Drug Research battery was examined by using linear mixed, logistic regression, and Cox proportional hazard models in relation to baseline and 3-year changes in serum TSH, free thyroxine (fT4), and free triiodothyronine (fT3). Results: Over 3 years, declining serum TSH was associated with reductions in fT4 and fT3, and an increased risk of incident cognitive impairment by 5 years (odds ratio1.77 [95% confidence interval: 1.19–2.61]; p = 0.004). A greater reduction in MMSE score was associated with larger TSH decline, at 3 (p = 0.001) and 5 years (p < 0.001), respectively. Steady fT4 concentrations were found in participants with rising TSH. Conclusions: In contrast to physiological expectation, in this group of 85-year-olds, a declining serum TSH was associated with reductions in free thyroid hormones over time. A decreasing serum TSH trajectory over time anticipated cognitive decline in later life. Declining TSH concentrations are a biomarker for cognitive impairment in later life.