Screening for subclinical thyroid dysfunction does not appear to be useful in the prevention of cognitive decline or dementia, Australian research shows.
Findings from a study published in JAMA Internal Medicine suggest existing clinical guidelines that recommend screening of subclinical thyroid dysfunction for prevention of cognitive decline or dementia should be revisited.
The study comprised an analysis of individual participant data from more than 74,000 adults across 23 cohorts investigating thyroid function and cognitive function and/or dementia.
At baseline, 80% of participants were euthyroid, 0.8% overtly hyperthyroid, 3.4% subclinically hyperthyroid, 5.6% subclinically hypothyroid, and 0.9% overtly hypothyroid.
Median follow-up ranged from 3.8 to 15.3 years, accumulating 525,222 person-years.
The study found thyroid dysfunction was not associated with global cognitive function or annual change in global cognitive function during follow-up.
“Participants with overt hypothyroidism had 0.11 standardised mean difference (95% CI, −0.01 to 0.23; P = .09) higher decline per year in global cognitive function than participants who were euthyroid, which translates to approximately 0.1 point on the MMSE scale faster decline per year based on the SD in the largest cohort for this analysis,” the study authors said.
The study found no association between continuous FT4 levels and global cognitive function.
It also found participants with overt hyperthyroidism had 0.20 standardised mean difference (95% CI, 0.07 to 0.33; P = .002) higher executive function score compared with participants who were euthyroid, but the association disappeared when participants using thyroid medication were removed.
“In both executive function and memory, participants with subclinical hypothyroidism performed better than participants who were euthyroid (executive function: 0.07 standardised mean difference; 95% CI, 0.01 to 0.13; P = .03; memory: 0.08 standardised mean difference; 95% CI, 0.01 to 0.15; P = .03).”
In longitudinal analyses, no association was found between thyroid dysfunction and incident dementia. The hazard ratio of dementia ranged from 1.54 (95% CI, 0.76 to 3.10) for overt hyperthyroidism to 0.79 (95% CI, 0.48 to 1.28) for overt hypothyroidism.
The investigators, including University of Western Australia’s Professor Leon Fricker and Professor Bu Yeap, said the study provided the strongest observational evidence to date that subclinical hypothyroidism was not associated with cognitive function or cognitive decline.
“Hence, it is unlikely that treatment for otherwise undetected subclinical thyroid dysfunction would improve cognitive function,” they said.
“Moreover, the chance of overtreatment is considerable, which increases the risk of atrial fibrillation, atherosclerosis, and cerebral infarction and thereby might increase the risk of cognitive decline.”