Is a mildly overactive thyroid linked to dementia in older adults?
Older Brazilians whose blood tests showed a mildly overactive thyroid were about four times as likely to have dementia, mainly vascular dementia, as those with normal thyroid function.
Source
Subclinical hyperthyroidism and dementia: the Sao Paulo Ageing & Health Study (SPAH)
Study at a glance
- Design
- Cross-sectional — Cross-sectional analysis of the baseline wave of a door-to-door population cohort, with logistic regression of dementia on thyroid status and TSH/FT4 quintiles, adjusted for age and BMI.
- N
- N=1276 · 1,276 participants aged 65+ with thyroid tests and no overt thyroid disease or thyroid medication; of these, 33 had subclinical hyperthyroidism and 49 had dementia.
- Population
- Adults aged 65 and over living in deprived census areas (shantytowns and Family Health Program households) of Butantan, Sao Paulo, Brazil
- Outcome
- Presence of dementia (any type), Alzheimer's disease and vascular dementia diagnosed with the 10/66 Dementia Research Group protocol
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What they did
The researchers used baseline data from the Sao Paulo Ageing & Health Study, which recruited people aged 65 and over by knocking on doors in a poor area of the city. After excluding people without thyroid tests or with overt thyroid disease or thyroid medication, 1,276 participants remained. Fasting blood was tested for TSH and free thyroxine, and dementia and its subtype were diagnosed with a standardized one-phase protocol designed for low- and middle-income countries. Logistic regression compared dementia odds in people with subclinical hyperthyroidism against those with normal thyroid function, adjusting for age and BMI.
What they found
Of the 1,276 participants, 33 had subclinical hyperthyroidism and 49 had dementia. After age adjustment, subclinical hyperthyroidism was associated with dementia of any type (odds ratio 4.1, 95% CI 1.3-13.1) and with vascular dementia (OR 5.3), but not with Alzheimer's disease overall; results held after adding BMI. In men only, it was linked to dementia (OR 8.0) and Alzheimer's disease (OR 12.4). Subclinical hypothyroidism showed no association, and the lowest TSH quintile carried higher dementia odds (OR 3.6) only when values below the normal range were included.
The limits
What it doesn't show
Because thyroid status and dementia were measured at the same time, the study cannot tell whether thyroid changes came first or whether dementia or its causes altered thyroid function. The key exposure group had only 33 people and there were 49 dementia cases, so the confidence intervals are very wide (the Alzheimer's estimate in men runs from 1.2 to 128.4) and the authors say chance cannot be excluded. Adjustment was limited to age and BMI, lifetime iodine intake was unknown, and about a third of the original cohort had no thyroid data because testing started after recruitment began.
Key terms
- Subclinical hyperthyroidism
- A blood-test pattern of low TSH with normal free thyroxine: the thyroid is mildly overactive but not enough to raise hormone levels out of range.
- TSH (thyroid-stimulating hormone)
- A pituitary hormone that rises when thyroid output is low and falls when it is high, making it the most sensitive screening test for thyroid function.
- Odds ratio
- How much higher the odds of an outcome are in one group than a reference group; an OR of 4 means roughly four times the odds.
- Vascular dementia
- Dementia caused mainly by damage to the brain's blood supply, such as strokes or small-vessel disease, as opposed to Alzheimer's pathology.
- Cross-sectional study
- A study that measures exposure and outcome at the same moment, so it can show associations but not which came first.
Flashcards
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Quiz yourself
What laboratory pattern defines subclinical hyperthyroidism?
Common questions
Does this study show that an overactive thyroid causes dementia?
No. Thyroid tests and dementia were assessed at the same time, so the association could run in either direction or reflect shared causes such as cardiovascular disease. Prospective studies are needed to establish timing.
Why might the link be strongest for vascular dementia in this population?
The authors point to Brazil's high burden of stroke and cardiovascular disease, especially among low-income people; excess thyroid hormone can promote problems like atrial fibrillation, which could feed into vascular brain damage.
Why are some odds ratios so large and uncertain?
Only 33 people had subclinical hyperthyroidism and even fewer of them had dementia, so each extra case moves the estimate a lot. That is why the interval for Alzheimer's disease in men stretches over two orders of magnitude.
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